UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 4)

Quantum Fuel Systems Technologies Worldwide, Inc.

(Name of Issuer)

Common Stock, $0.02 par value

(Title of Class of Securities)

74765E109

(CUSIP Number)

Ryan J. York

Davis Wright Tremaine LLP

1201 Third Avenue, Suite 2200

Seattle, WA 98101

(206) 622-3150

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

March 22, 2016

(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 (“Act”) or otherwise subject to the liabilities of that section of Act but shall be subject to all other provisions of the Act (however, see the Notes).

Amendment No. 4 to Schedule 13D

This Amendment No. 4 (“Amendment”) amends and supplements the Schedule 13D filed with the SEC on September 27, 2013, as previously amended by the Schedule 13D/A filed with the SEC on January 17, 2014, as previously amended by the Schedule 13D/A filed with the SEC on July 2, 2015, and as previously amended by the Schedule 13D/A filed with the SEC on December 29, 2015 (the “Schedule 13D”). Except as set forth in this Amendment, the Schedule 13D remains unchanged. Unless otherwise indicated, all capitalized terms used herein but not defined herein shall have the same meaning as set forth in the Schedule 13D.

The following items of the Schedule 13D are hereby amended as follows:

The Filers hereby add the following supplemental disclosure to Item 4:

DIP Credit Agreement

On March 22, 2016, the Issuer filed voluntary petitions for relief under Chapter 11 of the Bankruptcy Code in the U.S. Bankruptcy Court for the Central District of California (the “Bankruptcy Court,” and the filings therein, the “Chapter 11 Filings”). In connection with the Chapter 11 Filings, the Issuer will be seeking approval of debtor-in-possession financing on the terms set forth in the Superpriority Debtor-In-Possession Credit Agreement (the “DIP Credit Agreement”) between the Issuer and Douglas Acquisitions LLC (“Douglas Acquisitions”), upon entry of an interim court order approving the DIP Credit Agreement. Douglas Acquisitions was formed by two of the Filers, the K&M Douglas Trust and the James Douglas and Jean Douglas Irrevocable Descendants’ Trust. Subject to certain exceptions including the first lien position held by the Issuer’s pre-petition lender, the DIP Credit Agreement provides for senior secured, super-priority debtor-in-possession revolving credit facility with loans in an aggregate amount at any time outstanding of up to $6.0 million. The facility provided for under the DIP Credit Agreement will become available upon the satisfaction of customary conditions precedent, including the entry of an order of the Bankruptcy Court approving the DIP Credit Agreement. Subject to the satisfaction of the conditions set forth in the DIP Credit Agreement, the amount available to be borrowed under the DIP Credit Agreement will be made available to the Issuer through weekly advances pursuant to an approved budget. Amounts due under the DIP Financing bear interest at a rate of 7.5% per annum, payable in arrears on the last business day of each month. Upon the occurrence of an event of default, the interest rate increases to 9.5% per annum. Douglas Acquisitions received a commitment fee equal to 1% of the total amount of the DIP financing available under the DIP Credit Agreement.

The maturity date of the DIP financing (the “Termination Date”) is the earliest of: (i) the date that is 91 days after the closing date of the DIP financing, (ii) the date of the occurrence of an event of default under the DIP Credit Agreement, and (iii) the date the Issuer pays all amounts due and owing to Douglas Acquisitions under the DIP Credit Agreement. The DIP Credit Agreement contains certain customary mandatory repayment events, such as Issuer’s receipt of proceeds

from a sale of assets outside the ordinary case of business and insurance, casualty and condemnations claims. The DIP Credit Agreement also contains customary events of default, the occurrence of which could result in the acceleration of the Issuer’s obligation to repay the outstanding indebtedness under the DIP Credit Agreement.

Subject to certain exceptions including the first lien position held by the Issuer’s pre-petition lender, the Issuer’s obligations under the DIP Credit Agreement will be secured by a senior security interest in, and lien on, substantially all of the assets of the Issuer.

The foregoing description of the DIP Credit Agreement is qualified in its entirety by reference to the full text of the DIP Credit Agreement, a copy of which is attached to the Issuer’s Current Report on Form 8-K filed with the Securities and Exchange Commission on March 23, 2016, as Exhibit 10.1 and incorporated herein by reference.

The information disclosed in Item 4 above is incorporated herein by reference.

The Filers hereby add the following disclosure to this Item 7:

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.