SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 13D

[Rule 13d-101]

INFORMATION TO BE INCLUDED IN STATEMENTS FILED

PURSUANT TO § 240.13d-1(a) AND AMENDMENTS

THERETO FILED PURSUANT TO § 240.13d-2(a)

(Amendment No. 4)*

Navistar International Corporation

(Name of Issuer)

Common Stock, par value $0.10 per share

(Title of Class of Securities)

63934E108

(CUSIP Number)

Janet Yeung

MHR Fund Management LLC

40 West 57th Street #24

New York, New York 10019

(212) 262-0005

(Name, Address and Telephone Number of Person Authorized to Receive Notices of Communication)

October 24, 2012

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the Schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (the “Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(Continued on following pages)

(Page 1 of 12 Pages)

13D

13D

13D

13D

Explanatory Note

This statement on Schedule 13D (this “Statement”) amends and supplements, as Amendment No. 4, the Schedule 13D filed on June 15, 2012 (the “Initial Schedule 13D”), which was amended on June 25, 2012 by Amendment No. 1 to the Initial Schedule 13D (“Amendment No. 1”), on July 10, 2012 by Amendment No. 2 to the Initial Schedule 13D (“Amendment No. 2”), on October 9, 2012 by Amendment No. 3 to the Initial Schedule 13D (“Amendment No. 3” and, together with Amendment No. 2, Amendment No. 1 and the Initial Schedule 13D, the “Schedule 13D”) and relates to shares of common stock, par value $0.10 per share (the “Common Stock”), of Navistar International Corporation (the “Issuer”). Capitalized terms used in this Statement but not defined herein shall have the respective meanings given to such terms in Amendment No. 3.

Item 3 is hereby amended by adding the following paragraph:

On October 24, 2012, Master Account, Capital Partners (100) and Institutional Partners III acquired an aggregate of 1,598,000 shares of Common Stock in an underwritten public offering by the Issuer (the “Public Offering”) for aggregate consideration (excluding commissions) of approximately $29,962,500. All of the funds required to acquire such shares were obtained from the capital of Master Account, Capital Partners (100) and Institutional Partners III.

Item 4 is hereby amended by adding the following:

After conversations between representatives of the Reporting Persons and the Issuer, Mark H. Rachesky, M.D. entered into a Lock-up Agreement, dated October 23, 2012 (“Lock-up Agreement”) in connection with the Public Offering. A copy of the Lock-up Agreement is attached hereto as Exhibit 1 and incorporated herein by reference.

Item 5 is hereby amended by deleting such item in its entirety and replacing it with the following:

The percentages set forth below are calculated based on information contained in the Issuer’s Prospectus Supplement dated October 24, 2012, which disclosed that, after giving effect to the Public Offering there were 79,257,293 shares of Common Stock outstanding as of September 30, 2012.

(a) (i) Master Account may be deemed to be the beneficial owner of 907,167 shares of Common Stock held for its own account (approximately 1.1% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act).

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(ii) Capital Partners (100) may be deemed to be the beneficial owner of 121,163 shares of Common Stock held for its own account (approximately 0.2% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act).

(iii) Advisors may be deemed to be the beneficial owner of 1,028,330 shares of Common Stock (approximately 1.3% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act). This number consists of (A) 907,167 shares of Common Stock held for the account of Master Account and (B) 121,163 shares of Common Stock held for the account of Capital Partners (100).

(iv) Institutional Partners III may be deemed to be the beneficial owner of 10,844,670 shares of Common Stock held for its own account (approximately 13.7% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act).

(v) Institutional Advisors III may be deemed to be the beneficial owner of 10,844,670 shares of Common Stock (approximately 13.7% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act). This number consists of 10,844,670 shares of Common Stock held for the account of Institutional Partners III.

(vi) Fund Management may be deemed to be the beneficial owner of 11,873,000 shares of Common Stock (approximately 14.98% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act). This number consists of all of the shares of Common Stock otherwise described in this Item 5 by virtue of Fund Management’s investment management agreement with Master Account, Capital Partners (100) and Institutional Partners III.

(vii) MHR Holdings may be deemed to be the beneficial owner of 11,873,000 shares of Common Stock (approximately 14.98% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act). This number consists of all of the shares of Common Stock otherwise described in this Item 5 by virtue of MHR Holdings’ position as the managing member of Fund Management.

(viii) Dr. Rachesky may be deemed to be the beneficial owner of 11,873,000 shares of Common Stock (approximately 14.98% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d—3(d)(1)(i) under the Act). This number consists of all of the shares of Common Stock otherwise described in this Item 5 by virtue of Dr. Rachesky’s position as the managing member of each of Advisors, Institutional Advisors III and MHR Holdings.

(b) (i) Master Account may be deemed to have (x) the sole power to direct the disposition of 907,167 shares of Common Stock which may be deemed to be beneficially owned by Master Account as described above, and (y) the sole power to direct the voting of 907,167 shares of Common Stock which may be deemed to be beneficially owned by Master Account as described above.

(ii) Capital Partners (100) may be deemed to have (x) the sole power to direct the disposition of 121,163 shares of Common Stock which may be deemed to be beneficially owned by Capital Partners (100) as described above, and (y) the sole power to direct the voting of 121,163 shares of Common Stock which may be deemed to be beneficially owned by Capital Partners (100) as described above.

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(iii) Advisors may be deemed to have (x) the sole power to direct the disposition of 1,028,330 shares of Common Stock which may be deemed to be beneficially owned by Advisors as described above, and (y) the sole power to direct the voting of 1,028,330 shares of Common Stock which may be deemed to be beneficially owned by Advisors as described above.

(iv) Institutional Partners III may be deemed to have (x) the sole power to direct the disposition of 10,844,670 shares of Common Stock which may be deemed to be beneficially owned by Institutional Partners III as described above, and (y) the sole power to direct the voting of 10,844,670 shares of Common Stock which may be deemed to be beneficially owned by Institutional Partners III as described above.

(v) Institutional Advisors III may be deemed to have (x) the sole power to direct the disposition of 10,844,670 shares of Common Stock which may be deemed to be beneficially owned by Institutional Advisors III as described above, and (y) the sole power to direct the voting of 10,844,670 shares of Common Stock which may be deemed to be beneficially owned by Institutional Advisors III as described above.

(vi) Fund Management may be deemed to have (x) the sole power to direct the disposition of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by Fund Management as described above, and (y) the sole power to direct the voting of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by Fund Management as described above.

(vii) MHR Holdings may be deemed to have (x) the sole power to direct the disposition of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by MHR Holdings as described above, and (y) the sole power to direct the voting of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by MHR Holdings as described above.

(viii) Dr. Rachesky may be deemed to have (x) the sole power to direct the disposition of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by Dr. Rachesky as described above, and (y) the sole power to direct the voting of 11,873,000 shares of Common Stock which may be deemed to be beneficially owned by Dr. Rachesky as described above.

(d) (i) The partners of Master Account, including Advisors, have the right to participate in the receipt of dividends from, or proceeds from the sale of, Common Stock held for the account of Master Account in accordance with their partnership interests in Master Account.

(ii) The partners of Capital Partners (100), including Advisors, have the right to participate in the receipt of dividends from, or proceeds from the sale of, Common Stock held for the account of Capital Partners (100) in accordance with their partnership interests in Capital Partners (100).

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(iii) The partners of Institutional Partners III, including Institutional Advisors III, have the right to participate in the receipt of dividends from, or proceeds from the sale of, Common Stock held for the account of Institutional Partners III in accordance with their partnership interests in Institutional Partners III.

(e) Not Applicable.

Item 6 is hereby amended by adding the following:

The information set forth in Item 4 above and Exhibit 1 to this Statement are incorporated into this Item 6 by reference.

Item 7. Material to be filed as Exhibits.

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SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certify that the information set forth in this Statement is true, complete and correct.

Dated: October 26, 2012

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Annex A

Page 12 of 12

Exhibit 1

LOCK-UP AGREEMENT

October 23, 2012

J. P. Morgan Securities LLC

Goldman, Sachs & Co.

As Representatives of the

several Underwriters listed

in Schedule 1 hereto

c/o J. P. Morgan Securities LLC

383 Madison Avenue

New York, New York 10179

Goldman, Sachs & Co.

200 West Street

New York, New York 10282

Ladies and Gentlemen:

The undersigned understands that you, as Representatives of the several Underwriters, propose to enter into an Underwriting Agreement (the “Underwriting Agreement”) with Navistar International Corporation, a Delaware corporation (the “Company”), providing for the public offering (the “Public Offering”) by the several Underwriters named in Schedule 1 to the Underwriting Agreement (the “Underwriters”), of common stock, par value $0.10 per share, of the Company (the “Common Stock”). Capitalized terms used herein and not otherwise defined shall have the meanings set forth in the Underwriting Agreement.

In consideration of the Underwriters’ agreement to purchase and make the Public Offering of the Common Stock, and for other good and valuable consideration receipt of which is hereby acknowledged, the undersigned hereby agrees that, without the prior written consent of J. P. Morgan Securities LLC and Goldman, Sachs & Co. on behalf of the Underwriters, the undersigned will not, during the period ending 90 days after the date of the prospectus relating to the Public Offering (the “Prospectus”), (1) offer, pledge (other than a pledge arising from such shares being held in one or more brokerage accounts as set forth in the related account agreements; provided that such pledge does not result in a filing with the Commission under Section 13(d) or Section 16(a) of Securities Exchange Act of 1934, as amended), sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock, or any securities convertible into or exercisable or exchangeable for Common Stock, or publicly disclose the intention to make any such offer, sale, pledge or disposition, (2) enter into any swap or other agreement that transfers, in whole or in part, any of the economic consequences of ownership of the Common Stock or such other securities, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise or (3) make any demand for or exercise any right with respect to the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock, in each case other than (A) the Common Stock to be sold by the undersigned pursuant to the Underwriting Agreement, (B) transfers of shares of Common Stock as a bona fide gift or gifts, and (C) distributions of shares of

Common Stock to members or stockholders of the undersigned; provided that in the case of any transfer or distribution pursuant to clause (B) or (C), each donee or distributee shall execute and deliver to the Representatives a lock-up letter in the form of this paragraph; and provided, further, that in the case of any transfer or distribution pursuant to clause (B) or (C), no filing by any party (donor, donee, transferor or transferee) under the Securities Exchange Act of 1934, as amended, or other public announcement shall be required or shall be made voluntarily in connection with such transfer or distribution (other than a filing on a Form 5 made after the expiration of the 90-day period referred to above).

If the undersigned is an officer or director of the Company, (i) J.P. Morgan Securities LLC and Goldman, Sachs & Co. on behalf of the Underwriters agree that, at least three business days before the effective date of any release or waiver of the foregoing restrictions in connection with a transfer of shares of Common Stock, J.P. Morgan Securities LLC and Goldman, Sachs & Co. on behalf of the Underwriters will notify the Company of the impending release or waiver, and (ii) the Company has agreed in the Underwriting Agreement to announce the impending release or waiver by press release through a major news service at least two business days before the effective date of the release or waiver. Any release or waiver granted by J.P. Morgan Securities LLC and Goldman, Sachs & Co. on behalf of the Underwriters hereunder to any such officer or director shall only be effective two business days after the publication date of such press release. The provisions of this paragraph will not apply if (a) the release or waiver is effected solely to permit a transfer not for consideration and (b) the transferee has agreed in writing to be bound by the same terms described in this letter to the extent and for the duration that such terms remain in effect at the time of the transfer.

Nothing contained herein shall apply to or otherwise restrict any transfer or deemed transfer of shares of Common Stock or options to acquire Common Stock to the Company in connection with the exercise of options to acquire Common Stock, including, but not limited to, the payment of the exercise price of such options to acquire Common Stock, or to otherwise satisfy tax withholding obligations in connection with the vesting of Common Stock subject to transfer restrictions or in connection with the exercise of stock options to acquire shares of Common Stock.

In furtherance of the foregoing, the Company, and any duly appointed transfer agent for the registration or transfer of the securities described herein, are hereby authorized to decline to make any transfer of securities if such transfer would constitute a violation or breach of this Letter Agreement.

The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Letter Agreement. All authority herein conferred or agreed to be conferred and any obligations of the undersigned shall be binding upon the successors, assigns, heirs or personal representatives of the undersigned.

The undersigned understands that, if the Underwriting Agreement does not become effective prior to November 30, 2012, or if the Underwriting Agreement (other than the provisions thereof which survive termination) shall terminate or be terminated prior to payment for and delivery of the Common Stock to be sold thereunder, the undersigned shall be released from, all obligations under this Letter Agreement. The undersigned understands that the Underwriters are entering into the Underwriting Agreement and proceeding with the Public Offering in reliance upon this Letter Agreement.

This letter only applies to the undersigned and does not apply to, and shall not be deemed to apply to: (i) any affiliated fund of the undersigned; or (ii) any action taken prior to or after the date hereof by such affiliated fund.

This Letter Agreement and any claim, controversy or dispute arising under or related to this Letter Agreement shall be governed by and construed in accordance with the laws of the State of New York, without regard to the conflict of laws principles thereof.

[Signature page follows.]

Very truly yours,

By: /s/ Mark H. Rachesky                    

Print Name: Mark H. Rachesky

[Signature Page to the Lock-Up Agreement]