Exhibit (a)(1)(iv)

Offer to Purchase for Cash

All Outstanding Shares of Common Stock

of

GenMark Diagnostics, Inc.

at

$24.05 Net per Share

Pursuant to the Offer to Purchase

Dated March 25, 2021

by

Geronimo Acquisition Corp.

a wholly owned subsidiary of

Roche Holdings, Inc.

THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, EASTERN TIME,

AT THE END OF THE DAY ON WEDNESDAY, APRIL 21, 2021, UNLESS THE OFFER IS EXTENDED OR EARLIER TERMINATED.

March 25, 2021

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

We have been engaged by Geronimo Acquisition Corp., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Roche Holdings, Inc., a Delaware corporation (“Parent”), to act as the information agent (the “Information Agent”) in connection with Purchaser’s offer to purchase all outstanding shares of common stock, par value $0.0001 per share (the “Shares”), of GenMark Diagnostics, Inc., a Delaware corporation (“GenMark”), at a purchase price of $24.05 per Share (the “Offer Price”), net to the seller in cash, without interest, subject to any required withholding of taxes and upon the terms and subject to the conditions set forth in the Offer to Purchase dated March 25, 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 whose accounts you hold Shares registered in your name or in the name of your nominee.

Enclosed herewith for your information and forwarding to your clients for whose accounts you hold Shares registered in your name or in the name of your nominee are copies of the following documents:

1. The Offer to Purchase.

2. The related 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 and instructions providing information relating to federal income tax backup withholding. Facsimile copies of the Letter of Transmittal may be used to tender Shares.

3. Notice of Guaranteed Delivery to be used to accept the Offer if certificates for Shares and all other required documents cannot be delivered to Citibank, N.A., the depositary for the Offer (the “Depositary”), or if the procedures for book-entry transfer cannot be completed, prior to the expiration of the Offer.

4. A form of letter which 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. GenMark’s Solicitation/Recommendation Statement on Schedule 14D-9 dated March 25, 2021.

YOUR PROMPT ACTION IS REQUESTED. WE URGE YOU TO CONTACT YOUR CLIENTS AS PROMPTLY AS POSSIBLE. PLEASE NOTE THAT THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, EASTERN TIME, AT THE END OF WEDNESDAY, APRIL 21, 2021, UNLESS THE OFFER IS EXTENDED.

The Offer is being made pursuant to the Agreement and Plan of Merger, dated as of March 12, 2021 (the “Merger Agreement”), among GenMark, Parent and Purchaser. The Merger Agreement provides, among other things, that as soon as practicable after consummation of the Offer and subject to the satisfaction or waiver of the other conditions set forth in the Merger Agreement, Purchaser will merge with and into GenMark (the “Merger”), with GenMark continuing as the surviving corporation and a wholly owned subsidiary of Parent. At the effective time of the Merger, each outstanding Share (other than shares held by GenMark, any of its subsidiaries, Parent, Purchaser or any other subsidiary of Parent, or any stockholders who have properly exercised their appraisal rights under Section 262 of the Delaware General Corporation Law (the “DGCL”)) will be converted into the right to receive the Offer Price in cash, without interest and subject to deduction for any required withholding taxes. The Merger Agreement is more fully described in Section 13 of the Offer to Purchase.

The Board of Directors of GenMark (the “GenMark Board”) has duly and unanimously (i) approved, adopted and declared advisable the Merger Agreement and the transactions contemplated thereby, including the Offer and the Merger; (ii) determined that the transactions contemplated by the Merger Agreement, including the Offer and the Merger, are in the best interests of GenMark and its stockholders; (iii) agreed that the Merger Agreement will be effected under Section 251(h) of the DGCL; and (iv) resolved to recommend that GenMark’s stockholders accept the Offer and tender their Shares to Purchaser pursuant to the Offer. GenMark has been advised that all of its directors and executive officers intend to tender all of their transferrable Shares pursuant to the Offer.

The Offer is not subject to any financing condition. The conditions to the Offer are described in Sections 15 and 16 of the Offer to Purchase.

Purchaser will not pay any fees or commissions to any broker, dealer or any other person (other than to the Information Agent and the Depositary as described in the Offer to Purchase) 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 reasonable and necessary costs and expenses incurred by them in forwarding the enclosed materials to their clients.

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.

In order to take advantage of the Offer, a duly executed and properly completed Letter of Transmittal (or a manually signed facsimile thereof), or an Agent’s Message (as defined in the Offer to Purchase) in connection with a book-entry transfer of Shares, and any other required documents, should be sent to the Depositary, and certificates representing the tendered Shares should be delivered or such Shares should be tendered by book-entry transfer, all in accordance with the instructions contained in the Letter of Transmittal and the Offer to Purchase.

If holders of Shares wish to tender, but it is impracticable for them to forward their certificates or other required documents or to complete the procedures for delivery by book-entry transfer prior to the expiration of the Offer, a tender may be effected by following the guaranteed delivery procedures described in Section 3 of the Offer to Purchase.

Any inquiries you may have with respect to the Offer should be addressed to, and additional copies of the enclosed materials may be obtained from, the undersigned or the Dealer Manager at their respective addresses and telephone numbers set forth on the back cover of the Offer to Purchase.

Very truly yours,

MacKenzie Partners, Inc.

NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS SHALL RENDER YOU THE AGENT OF PARENT, PURCHASER, THE INFORMATION AGENT, THE DEALER MANAGER 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 DOCUMENTS ENCLOSED HEREWITH AND THE STATEMENTS CONTAINED THEREIN.