X0202 SCHEDULE 13G/A 0001224962 XXXXXXXX LIVE 1 Common Stock, par value $0.001 per share 03/31/2026 0001501697 X4 PHARMACEUTICALS, INC. 98420X202 61 North Beacon Street, 4th Floor Boston MA 02134 Rule 13d-1(c) Perceptive Advisors LLC b DE 0 6889163 0 6889163 6889163 N 7.3 IA Joseph Edelman b X1 0 6889163 0 6889163 6889163 N 7.3 IN Perceptive Life Sciences Master Fund, Ltd. b E9 0 6889163 0 6889163 6889163 N 7.3 CO X4 PHARMACEUTICALS, INC. 61 North Beacon Street, 4th Floor, Boston, Massachusetts The names of the persons filing this report (collectively, the "Reporting Persons") with respect to shares of Common Stock, par value $0.001 per share (the "Common Stock") of X4 PHARMACEUTICALS, INC. (the "Issuer") are: Perceptive Advisors LLC ("Perceptive Advisors") Joseph Edelman ("Mr. Edelman") Perceptive Life Sciences Master Fund, Ltd. (the "Master Fund") The address of the principal business office of each of the Reporting Persons is: 51 Astor Place, 10th Floor New York, NY 10003 Perceptive Advisors is a Delaware limited liability company Mr. Edelman is a United States citizen The Master Fund is a Cayman Islands corporation Y The information required by this item with respect to each Reporting Person is set forth in Rows 5 through 9 and 11 of the cover pages to this Schedule 13G. The ownership percentages reported are based on 94,319,696 outstanding shares of Common Stock as of May 1, 2026, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on May 6, 2026. Neither Perceptive Advisors nor Mr. Edelman directly hold any shares of Common Stock or any Warrants. The Master Fund directly holds 6,737,648 shares of Common Stock and 151,515 Warrants immediately exercisable for shares of Common Stock, subject to the Beneficial Ownership Limitation (as defined below). The terms of the Warrants provide that the Warrants may not be exercised if, after such exercise, the Reporting Persons would beneficially own, as determined in accordance with Section 13(d) if the Securities Exchange Act of 1934, as amended, more than 9.99% of the shares of Common Stock then issued and outstanding (the "Beneficial Ownership Limitation"). As of the date hereof, the Beneficial Ownership Limitation permits the Reporting Persons to exercise Warrants for an aggregate of 151,515 shares of Common Stock. Perceptive Advisors serves as the investment manager to the Master Fund. Mr. Edelman is the managing member of Perceptive Advisors. Perceptive Advisors: 7.3% Mr. Edelman: 7.3% Master Fund: 7.3% Perceptive Advisors: 0 Mr. Edelman: 0 Master Fund: 0 Perceptive Advisors: 6,889,163 Mr. Edelman: 6,889,163 Master Fund: 6,889,163 Perceptive Advisors: 0 Mr. Edelman: 0 Master Fund: 0 Perceptive Advisors: 6,889,163 Mr. Edelman: 6,889,163 Master Fund: 6,889,163 Y Y Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. Perceptive Advisors LLC /s/ Joseph Edelman 05/15/2026 Joseph Edelman /s/ Joseph Edelman 05/15/2026 Perceptive Life Sciences Master Fund, Ltd. /s/ Joseph Edelman 05/15/2026