X4 Pharmaceuticals, Inc

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(AMENDMENT NO. )

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

X4 Pharmaceuticals, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

X4 PHARMACEUTICALS, INC.

61 North Beacon Street, 4 th Floor

Boston, Massachusetts 02134

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To Be Held On May 11, 2026

Dear Stockholder of X4 Pharmaceuticals, Inc.:

You are cordially invited to attend the 2026 Annual Meeting of Stockholders (the “Annual Meeting”) of X4 Pharmaceuticals, Inc., a Delaware corporation (the “Company”) on Monday, May 11, 2026 at 12:00 p.m. Eastern Time. The Annual Meeting will be held virtually via live audio-only webcast at www.virtualshareholdermeeting.com/XFOR2026. The Annual Meeting will be held online only. The Annual Meeting will be held for the following purposes:

1. To elect the three Class III director nominees named in the accompanying proxy statement (the “Proxy Statement”), to serve until the 2029 Annual Meeting of Stockholders and until their successors are duly elected and qualified.

2. To ratify the selection of PricewaterhouseCoopers LLP as the Company’s independent registered public accounting firm for the year ending December 31, 2026.

3. To approve the Second Amended and Restated 2017 Equity Incentive Plan to increase the number of shares available for issuance thereunder.

4. To approve, on a non-binding, advisory basis, the compensation of the Company’s named executive officers.

5. To conduct any other business as may properly come before the Annual Meeting or any adjournments or postponements thereof.

These items of business are more fully described in the Proxy Statement accompanying this Notice.

The Board of Directors has fixed March 13, 2026 as the record date for the Annual Meeting. Only stockholders of record at the close of business on that date will be entitled to notice of, and tovote at, the Annual Meeting or any adjournments or postponements thereof.

Instructions for accessing the virtual Annual Meeting are provided in the Proxy Statement. Unless otherwise announced differently at the meeting or on the meeting website, in the event of a technical malfunction or other situation that the meeting chair determines may affect the ability of the Annual Meeting to satisfy the requirements for a meeting of stockholders to be held by means of remote communication under the Delaware General Corporation Law, or that otherwise makes it advisable to adjourn the Annual Meeting, the meeting chair or secretary will convene the meeting at 12:00 p.m. Eastern Time on the date specified above and at the Company’s address specified above solely for the purpose of adjourning the meeting to reconvene at a date, time and physical or virtual location announced by the meeting chair or secretary. Under either of the foregoing circumstances, we will post information regarding the announcement on the Investors page of the Company’s website at https://investors.x4pharma.com/.

By Order of the Board of Directors,

David Kirske

Chief Financial Officer, Treasurer and Corporate Secretary

Boston, Massachusetts

You are cordially invited to attend the Annual Meeting. Whether or not you expect to attend the AnnualMeeting, please vote online or, if you requested printed copies of the proxy materials, by telephone or by completing, dating, signing and returning the proxy card or voting instruction form mailed to you, as promptly as possible in order to ensure your representation at the Annual Meeting. If you participate virtually in the annual meeting, you may vote at that time, even if you previously submitted your vote. Please note, however, that if your shares are held of record by a broker, fiduciary or custodian and you wish to vote at the Annual Meeting, you must obtain a proxy issued in your name from that record holder in order to vote your shares that are held in such agent’s name and account.

LEGAL MATTERS

Important Notice Regarding the Availability of Proxy Materials for the 2026 Annual Meeting of Stockholders to Be Held on May 11, 2026. The Proxy Statement and Annual Report for the year ended December 31, 2025 are available atwww.proxyvote.com.

Forward-Looking Statements. The Proxy Statement contains “forward-looking statements” within the meaning of the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995, which statements are subject to substantial risks and uncertainties and are based on estimates and assumptions. All statements other than statements of historical fact included in the Proxy Statement, including statements about the Company’s Board of Directors, corporate governance practices and executive compensation program and equity compensation utilization, are forward-looking statements. In some cases, you can identify forward-looking statements by terms such as “may,” “might,” “will,” “objective,” “intend,” “should,” “could,” “can,” “would,” “expect,” “believe,” “design,” “estimate,” “predict,” “potential,” “plan” or the negative of these terms, and similar expressions intended to identify forward-looking statements. These statements involve known and unknown risks, uncertainties and other factors that could cause our actual results to differ materially from the forward-looking statements expressed or implied in the Proxy Statement. Such risks, uncertainties and other factors include those risks described in “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in the Company’s most recent Annual Report on Form 10-K filed with the U.S. Securities and Exchange Commission (“SEC”) and other subsequent documents we file with the SEC. The Company expressly disclaims any obligation to update or alter any statements whether as a result of new information, future events or otherwise, except as required by law.

Website References. Website references throughout this document are inactive textual references and provided for convenience only, and the content on the referenced websites is not incorporated herein by reference and does not constitute a part of the Proxy Statement.

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