Sec Form 4 Filing - Lambert Danielle @ Horizon Quantum Holdings Ltd. - 2026-03-19

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Lambert Danielle 2. Issuer Name and Ticker or Trading Symbol Horizon Quantum Holdings Ltd. [ HQ] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O HORIZON QUANTUM HOLDINGS LTD., 29 MEDIA CIR. #05-22 3. Date of Earliest Transaction (MM/DD/YY) 03/19/2026 (Street) SINGAPORE138565 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Class A Ordinary Shares 03/19/2026( 1 ) P( 1 ) 84,602 A $ 11.82 84,602 I ( 2 ) Penchant Family Holdings LLC ( 2 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Lambert Danielle C/O HORIZON QUANTUM HOLDINGS LTD. 29 MEDIA CIR. #05-22 SINGAPORE138565 X Signatures /s/ Danielle Lambert 03/25/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On March 6, 2026, dMY Squared Technology Group, Inc. ("dMY"), the Company, and Horizon Quantum Computing Pte. Ltd. entered into a subscription agreement with Penchant Family Holdings LLC, whereby the Company, upon the closing of the Company's business combination with dMY, would issue 84,602 shares of the Company's Class A ordinary shares, with no par value (the "Class A Ordinary Shares") to Penchant Family Holdings LLC at a price per share of $11.82, for an aggregate purchase price of approximately $1,000,000. The closing of the Company's business combination occurred on March 19, 2026, and the 84,602 Class A Ordinary Shares of the Company were sold to Penchant Family Holdings LLC on that date. ( 2 )Penchant Family Holdings LLC is controlled by Penchant Holdings, Inc., its Managing Member, of which Danielle Lambert serves as its President.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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