X4 Pharmaceuticals, Inc

Rule 10b5-1 Trading Plans

During the quarter ended September 30, 2025, no director or Section 16 officer adopted, modified orterminated any Rule 10b5-1 trading arrangements or non-Rule 10b5-1 trading arrangements (in each case, as defined in Item 408(a) of Regulation S-K), except as described below:

In connection with her departure from the Company, Dr. Paula Ragan, the Company’sformer President and Chief Executive Officer and a former director, terminated a Rule 10b5-1 trading arrangement on August 14, 2025. The terminated Rule 10b5-1 trading arrangement was originally adopted on June 28, 2024, and provided for the sale of up to 313,794 shares of common stock and (ii)25,682 shares of our common stock to satisfy tax withholding obligation upon the vesting of performance-based restricted stock units.

In connection with his departure from the Company, Adam Mostafa, the Company’sformer Chief Financial Officer, terminated a Rule 10b5-1 trading arrangement on August 14, 2025. The terminated Rule 10b5-1 trading arrangement was originally adopted on June 28, 2024, and provided for the sale 11,229 shares of our common stock to satisfy tax withholding obligation upon the vesting of performance-based restricted stock units.

On September 4, 2025, Mark Baldry, the Company’sformer Chief Commercial Officer, adopted a written trading arrangement for the sale of Company securities intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). In connection with his departure from the Company, Mr. Baldry terminated the 10b5-1 trading arrangement on September 25, 2025. This trading arrangement was subject to a 90-day “cooling off” period before any trades could be conducted thereunder, would have expired on February 28, 2026 and provided for the sale of an indeterminable(1)number of shares of our common stock.

(1) The number of shares that were to be sold in accordance with these trading arrangements was indeterminable at the time of the filing of this Quarterly Report on Form 10-Q for the period ended September 30, 2025. Upon vesting of performance- based restricted stock units, a sell-to-cover trade would have occurred to satisfy any applicable tax withholding obligations on behalf of the employee. The number of shares sold in the sell-to-cover would have varied based on the market price of the Company’s common stock at the time of settlement and the employee’s minimum tax withholding obligation.