Exhibit (a)(1)(iv)

Offer To Purchase For Cash

All Outstanding Shares of Common Stock

of

VIELA BIO, INC.

at

$53.00 net per Share

Pursuant to the Offer to Purchase dated February 12, 2021

by

TEIRIPIC MERGER SUB, INC.,

a direct wholly owned subsidiary of

HORIZON THERAPEUTICS USA, INC.,

an indirect wholly owned subsidiary of

HORIZON THERAPEUTICS PUBLIC LIMITED COMPANY

THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE ONE MINUTE FOLLOWING

11:59 P.M. (EASTERN TIME), ON MARCH 12, 2021, UNLESS THE OFFER IS

EXTENDED OR EARLIER TERMINATED.

To Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees:

We have been engaged by Teiripic Merger Sub, Inc., a Delaware corporation (“Purchaser”) and a direct wholly owned subsidiary of Horizon Therapeutics USA, a Delaware corporation (“Parent”) and an indirect wholly owned subsidiary of Horizon Therapeutics plc, a public limited company organized under the laws of Ireland (“Ultimate Parent”), to act as information agent (the “Information Agent”) in connection with Purchaser’s offer to purchase all of the outstanding shares of common stock, par value $0.001 per share (the “Shares”), of Viela Bio, Inc., a Delaware corporation (“Viela”), that are issued and outstanding at a price of $53.00 per Share (the “Offer Price”), net to the holder thereof, in cash, without interest, subject to any applicable withholding taxes, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated February 12, 2021 (as it may be amended or supplemented from time to time, the “Offer to Purchase”), and the related Letter of Transmittal (as it may be amended or supplemented from time to time, the “Letter of Transmittal” and, together with the Offer to Purchase, the “Offer”) enclosed herewith. Please furnish copies of the enclosed materials to those of your clients for whom you hold Shares registered in your name or in the name of your nominee.

THE BOARD OF DIRECTORS OF VIELA HAS UNANIMOUSLY RECOMMENDED THAT

STOCKHOLDERS TENDER ALL OF THEIR SHARES PURSUANT TO THE OFFER.

The conditions of the Offer are described in Section 15 of the Offer to Purchase.

For your information and for forwarding to your clients for whom you hold Shares registered in your name or in the name of your nominee, we are enclosing the following documents:

1. The Offer to Purchase;

2. The Letter of Transmittal for your use in accepting the Offer and tendering Shares and for the information of your clients, together with the included IRS Form W-9;

3. A notice of guaranteed delivery to be used to accept the Offer if Shares and all other required documents are not immediately available or cannot be delivered to American Stock Transfer & Trust Company, LLC (the “Depositary”) by one minute following 11:59 p.m. (Eastern time), on March 12, 2021 (the “Expiration Date,” unless the period during which the Offer is open is extended pursuant to and in accordance with the Merger Agreement (as defined below), in which event “Expiration Date” will mean the subsequent time and date to which the expiration of the Offer is extended in accordance with the terms of the Merger Agreement) or if the procedure for book-entry transfer cannot be completed by the Expiration Date (the “Notice of Guaranteed Delivery”);

4. A form of letter that may be sent to your clients for whose accounts you hold Shares registered in your name or in the name of your nominee, with space provided for obtaining such clients’ instructions with regard to the Offer;

5. Viela’s Solicitation/Recommendation Statement on Schedule 14D-9; and

6. A return envelope addressed to the Depositary for your use only.

We urge you to contact your clients as promptly as possible. Please note that the Offer and withdrawal rights will expire at one minute following 11:59 p.m. (Eastern time), on March 12, 2021, unless the Offer is extended or earlier terminated in accordance with the terms of the Merger Agreement.

The Offer is being made pursuant to the Agreement and Plan of Merger, dated as of January 31, 2021 (as it may be amended or supplemented from time to time in accordance with its terms, the “Merger Agreement”), by and among Parent, Purchaser, Viela and, solely for purposes of Sections 6.7 and 9.12 thereof, Ultimate Parent, pursuant to which, as soon as practicable following the first time as of which Purchaser accepts any Shares for payment pursuant to the Offer (the “Offer Acceptance Time”) and subject to the satisfaction or waiver of the remaining conditions set forth in the Merger Agreement, Purchaser will be merged with and into Viela (the “Merger”), with Viela continuing as the surviving corporation in the Merger and as a direct wholly owned subsidiary of Parent. At the date and time of the filing of the certificate of merger in respect of the Merger with the Secretary of State of the State of Delaware or at such later date and time as is agreed upon in writing by the parties to the Merger Agreement and specified in the certificate of merger (the “Effective Time”), each Share then outstanding (other than any Shares owned by Parent, Purchaser or Viela or by any other wholly owned subsidiary of Parent, which shall be cancelled and retired and shall cease to exist, and no consideration shall be delivered in exchange therefor) will be cancelled and (other than any Shares held by holders who are entitled to appraisal rights under Section 262 of the DGCL and who have properly exercised and perfected their respective demands for appraisal of such shares in the time and manner provided in Section 262 of the DGCL and, as of the Effective Time, have neither effectively withdrawn nor lost their rights to such appraisal and payment under the DGCL) will be converted into the right to receive cash in an amount equal to the Offer Price, without interest, subject to any applicable withholding of taxes. Under no circumstances will interest be paid with respect to the purchase of Shares pursuant to the Offer, regardless of any extension of the Offer or any delay in making payment for Shares.

After careful consideration, the board of directors of Viela has unanimously (i) determined that the Merger Agreement and the transactions contemplated therein are advisable and fair to, and in the best interests of Viela and its stockholders; (ii) agreed that the Merger Agreement shall be subject to Section 251(h) of the DGCL; (iii) approved the execution, delivery and performance by Viela of the Merger Agreement and the consummation of the transactions contemplated therein; and (iv) resolved to recommend that the holders of Shares accept the Offer and tender their Shares to Purchaser pursuant to the Offer.

For Shares to be properly tendered pursuant to the Offer, (a) the share certificates or confirmation of receipt of such Shares under the procedure for book-entry transfer, together with a properly completed and duly executed Letter of Transmittal, including any required signature guarantees, or, in the case of book-entry transfers, either such Letter of Transmittal or an Agent’s Message (as defined in Section 3 of the Offer to Purchase) in lieu of such Letter of Transmittal, and any other documents required in the Letter of Transmittal, must be timely

received by the Depositary or (b) the tendering stockholder must comply with the guaranteed delivery procedures, all in accordance with the Offer to Purchase and the Letter of Transmittal. You may gain some additional time by making use of the Notice of Guaranteed Delivery.

Except as set forth in the Offer to Purchase, Purchaser will not pay any fees or commissions to any broker or dealer or other person for soliciting tenders of Shares pursuant to the Offer. Purchaser will, however, upon request, reimburse brokers, dealers, commercial banks, trust companies and other nominees for customary mailing and handling expenses incurred by them in forwarding the offering material to their customers. Purchaser will pay all stock transfer taxes applicable to its purchase of Shares pursuant to the Offer, subject to Instruction 6 of the Letter of Transmittal.

You may contact us as Information Agent with questions and requests for assistance, and for additional copies of the Offer to Purchase, the Letter of Transmittal, the Notice of Guaranteed Delivery and other tender offer materials, at the address and telephone number set forth on the back cover of the Offer to Purchase.

Very truly yours,

Alliance Advisors, LLC

Nothing contained herein or in the enclosed documents shall render you the agent of Purchaser, Parent, Viela, the Information Agent, or the Depositary or any affiliate of any of them or authorize you or any other person to use any document or make any statement on behalf of any of them in connection with the Offer other than the enclosed documents and the statements contained therein.