SCHEDULE 13G 0000918923 XXXXXXXX LIVE Common Stock, par value $0.001 per share 08/13/2025 0001501697 X4 Pharmaceuticals, Inc 98420X202 61 NORTH BEACON STREET 4TH FLOOR BOSTON MA 02134 Rule 13d-1(c) BIOTECHNOLOGY VALUE FUND L P a DE 0.00 1168649.00 0.00 1168649.00 1168649.00 N 5.2 PN BVF I GP LLC a DE 0.00 1168649.00 0.00 1168649.00 1168649.00 N 5.2 OO BIOTECHNOLOGY VALUE FUND II LP a DE 0.00 906855.00 0.00 906855.00 906855.00 N 4.0 PN BVF II GP LLC a DE 0.00 906855.00 0.00 906855.00 906855.00 N 4.0 OO Biotechnology Value Trading Fund OS LP a E9 0.00 123369.00 0.00 123369.00 123369.00 N 0.6 PN BVF Partners OS Ltd. a E9 0.00 123369.00 0.00 123369.00 123369.00 N 0.6 CO BVF GP HOLDINGS LLC a DE 0.00 2075504.00 0.00 2075504.00 2075504.00 N 9.2 OO BVF PARTNERS L P/IL a DE 0.00 2243659.00 0.00 2243659.00 2243659.00 N 9.9 IA PN BVF INC/IL a DE 0.00 2243659.00 0.00 2243659.00 2243659.00 N 9.9 CO LAMPERT MARK N a X1 0.00 2243659.00 0.00 2243659.00 2243659.00 N 9.9 IN X4 Pharmaceuticals, Inc 61 North Beacon Street, 4th Floor, Boston, Massachusetts 02134 Biotechnology Value Fund, L.P. ("BVF") BVF I GP LLC ("BVF GP") Biotechnology Value Fund II, L.P. ("BVF2") BVF II GP LLC ("BVF2 GP") Biotechnology Value Trading Fund OS LP ("Trading Fund OS") BVF Partners OS Ltd. ("Partners OS") BVF GP Holdings LLC ("BVF GPH") BVF Partners L.P. ("Partners") BVF Inc. Mark N. Lampert ("Mr. Lampert") Each of the foregoing is referred to as a "Reporting Person" and collectively as the "Reporting Persons." Biotechnology Value Fund, L.P. 44 Montgomery St., 40th Floor San Francisco, California 94104 BVF I GP LLC 44 Montgomery St., 40th Floor San Francisco, California 94104 Biotechnology Value Fund II, L.P. 44 Montgomery St., 40th Floor San Francisco, California 94104 BVF II GP LLC 44 Montgomery St., 40th Floor San Francisco, California 94104 Biotechnology Value Trading Fund OS LP PO Box 309 Ugland House Grand Cayman, KY1-1104 Cayman Islands BVF Partners OS Ltd. PO Box 309 Ugland House Grand Cayman, KY1-1104 Cayman Islands BVF GP Holdings LLC 44 Montgomery St., 40th Floor San Francisco, California 94104 BVF Partners L.P. 44 Montgomery St., 40th Floor San Francisco, California 94104 BVF Inc. 44 Montgomery St., 40th Floor San Francisco, California 94104 Mark N. Lampert 44 Montgomery St., 40th Floor San Francisco, California 94104 Biotechnology Value Fund, L.P. Delaware BVF I GP LLC Delaware Biotechnology Value Fund II, L.P. Delaware BVF II GP LLC Delaware Biotechnology Value Trading Fund OS LP Cayman Islands BVF Partners OS Ltd. Cayman Islands BVF GP Holdings LLC Delaware BVF Partners L.P. Delaware BVF Inc. Delaware Mark N. Lampert United States Y As of the close of business on August 20, 2025, the Reporting Persons and a certain Partners managed account (the "Partners Managed Account") held Pre-Funded Common Stock Purchase Warrants (the "Pre-Funded Warrants") exercisable for an aggregate of 1,288,288 shares of the Issuer's common stock, par value $0.001 per share (the "Shares"). The Pre-Funded Warrants are exercisable immediately, do not expire, and have an exercise price of $0.001 per Share. The Reporting Persons and the Partners Managed Account (together with their affiliates and other attribution parties) may not exercise any portion of the Pre-Funded Warrants they hold to the extent that, immediately after giving effect to such issuance after exercise, they would beneficially own more than 9.99% of the outstanding Shares (the "Pre-Funded Warrants Blocker"). As of the close of business on August 20, 2025, the Pre-Funded Warrants Blocker limits the exercise of the Pre-Funded Warrants held by the Reporting Persons and the Partners Managed Account to 9,917 out of 1,288,288 Shares underlying the Pre-Funded Warrants held by them. As of the close of business on August 20, 2025, (i) BVF beneficially owned 1,168,649 Shares, including 9,917 Shares underlying the Pre-Funded Warrants held by it and excluding 658,370 Shares underlying certain Pre-Funded Warrants held by it, (ii) BVF2 beneficially owned 906,855 Shares, excluding 523,019 Shares underlying Pre-Funded Warrants held by it, and (iii) Trading Fund OS beneficially owned 123,369 Shares, excluding 71,152 Shares underlying certain Pre-Funded Warrants held by it. BVF GP, as the general partner of BVF, may be deemed to beneficially own the 1,168,649 Shares beneficially owned by BVF. BVF2 GP, as the general partner of BVF2, may be deemed to beneficially own the 906,855 Shares beneficially owned by BVF2. Partners OS, as the general partner of Trading Fund OS, may be deemed to beneficially own the 123,369 Shares beneficially owned by Trading Fund OS. BVF GPH, as the sole member of each of BVF GP and BVF2 GP, may be deemed to beneficially own the 2,075,504 Shares beneficially owned in the aggregate by BVF and BVF2. Partners, as the investment manager of BVF, BVF2 and Trading Fund OS, and the sole member of Partners OS, may be deemed to beneficially own the 2,243,659 Shares beneficially owned in the aggregate by BVF, BVF2 and Trading Fund OS and held in the Partners Managed Account, including 44,786 Shares held in the Partners Managed Account, which excludes 25,830 Shares underlying the Pre-Funded Warrants held by it. BVF Inc., as the general partner of Partners, may be deemed to beneficially own the 2,243,659 Shares beneficially owned by Partners. Mr. Lampert, as a director and officer of BVF Inc., may be deemed to beneficially own the 2,243,659 Shares beneficially owned by BVF Inc. The foregoing should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership of any Shares owned by another Reporting Person. BVF GP disclaims beneficial ownership of the Shares beneficially owned by BVF. BVF2 GP disclaims beneficial ownership of the Shares beneficially owned by BVF2. Partners OS disclaims beneficial ownership of the Shares beneficially owned by Trading Fund OS. BVF GPH disclaims beneficial ownership of the Shares beneficially owned by BVF and BVF2. Each of Partners, BVF Inc. and Mr. Lampert disclaims beneficial ownership of the Shares beneficially owned by BVF, BVF2 and Trading Fund OS and held in the Partners Managed Account, and the filing of this statement shall not be construed as an admission that any such person or entity is the beneficial owner of any such securities. The following percentages are based upon a denominator that was calculated as follows: (i) 11,408,357 Shares outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on August 8, 2025, plus (ii) 11,040,776 Shares issued and sold by the Issuer pursuant to a Securities Purchase Agreement, dated August 12, 2025, as set forth in the Issuer's Current Report on Form 8-K filed with the Securities and Exchange Commission on August 13, 2025, plus (iii) 9,917 Shares issuable upon the exercise of certain Pre-Funded Warrants held by the Reporting Persons, as applicable. As of the close of business on August 20, 2025, (i) BVF beneficially owned approximately 5.2% of the outstanding Shares, (ii) BVF2 beneficially owned approximately 4.0% of the outstanding Shares, (iii) Trading Fund OS beneficially owned less than 1% of the outstanding Shares, (iv) BVF GP may be deemed to beneficially own approximately 5.2% of the outstanding Shares, (v) BVF2 GP may be deemed to beneficially own approximately 4.0% of the outstanding Shares, (vi) Partners OS may be deemed to beneficially own less than 1% of the outstanding Shares, (vii) BVF GPH may be deemed to beneficially own approximately 9.2% of the outstanding Shares, and (viii) each of Partners, BVF Inc. and Mr. Lampert may be deemed to beneficially own approximately 9.99% of the outstanding Shares (less than 1% of the outstanding Shares are held in the Partners Managed Account). See Cover Pages Items 5-9. See Cover Pages Items 5-9. See Cover Pages Items 5-9. See Cover Pages Items 5-9. Y N BVF GP, BVF GPH, Partners, BVF Inc. and Mr. Lampert share voting and dispositive power over the Shares beneficially owned by BVF. BVF GPH, Partners, BVF Inc. and Mr. Lampert share voting and dispositive power over the Shares beneficially owned by BVF2. Partners, BVF Inc. and Mr. Lampert share voting and dispositive power over the Shares beneficially owned by Trading Fund OS and held in the Partners Managed Account. Y N See Exhibit 99.1. 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. 99.1 - Joint Filing Agreement, August 20, 2025. BIOTECHNOLOGY VALUE FUND L P /s/ Mark N. Lampert 08/20/2025 BVF I GP LLC /s/ Mark N. Lampert 08/20/2025 BIOTECHNOLOGY VALUE FUND II LP /s/ Mark N. Lampert 08/20/2025 BVF II GP LLC /s/ Mark N. Lampert 08/20/2025 Biotechnology Value Trading Fund OS LP /s/ Mark N. Lampert 08/20/2025 BVF Partners OS Ltd. /s/ Mark N. Lampert 08/20/2025 BVF GP HOLDINGS LLC /s/ Mark N. Lampert 08/20/2025 BVF PARTNERS L P/IL /s/ Mark N. Lampert 08/20/2025 BVF INC/IL /s/ Mark N. Lampert 08/20/2025 LAMPERT MARK N /s/ Mark N. Lampert 08/20/2025