Sec Form 4 Filing - Humming Bird Ventures Management NV @ BillionToOne, Inc. - 2025-11-07

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 Humming Bird Ventures Management NV 2. Issuer Name and Ticker or Trading Symbol BillionToOne, Inc. [ BLLN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) SQUARE VICTORIA REGAIN 1 3. Date of Earliest Transaction (MM/DD/YY) 11/07/2025 (Street) SINT-JOOST-TEN-NODE, C91210 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ 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 Common Stock 11/07/2025 C( 1 )( 2 ) 640,727 A 640,727 I See Footnotes ( 3 ) ( 8 ) Common Stock 11/07/2025 C( 1 )( 2 ) 18,845 A 18,845 I See Footnotes ( 4 ) ( 8 ) Common Stock 11/07/2025 C( 1 )( 2 ) 2,015,118 A 2,015,118 I See Footnotes ( 5 ) ( 8 ) Common Stock 11/07/2025 C( 1 )( 2 ) 2,451,665 A 2,631,664 I See Footnotes ( 6 ) ( 8 ) Common Stock 11/07/2025 C( 1 )( 2 ) 2,580,833 A 2,580,833 I See Footnotes ( 7 ) ( 8 ) Common Stock 11/07/2025 J( 9 ) 640,727 D 0 I See Footnotes ( 3 ) ( 8 ) Common Stock 11/07/2025 J( 9 ) 18,845 D 0 I See Footnotes ( 4 ) ( 8 ) Common Stock 11/07/2025 J( 9 ) 2,015,118 D 0 I See Footnotes ( 5 ) ( 8 ) Common Stock 11/07/2025 J( 9 ) 2,631,664 D 0 I See Footnotes ( 6 ) ( 8 ) Common Stock 11/07/2025 J( 9 ) 2,580,833 D 0 I See Footnotes ( 7 ) ( 8 ) Class A Common Stock 11/07/2025 J( 9 ) 640,727 A 640,727 I See Footnotes ( 3 ) ( 8 ) Class A Common Stock 11/07/2025 J( 9 ) 18,845 A 18,845 I See Footnotes ( 4 ) ( 8 ) Class A Common Stock 11/07/2025 J( 9 ) 2,015,118 A 2,015,118 I See Footnotes ( 5 ) ( 8 ) Class A Common Stock 11/07/2025 J( 9 ) 2,631,664 A 2,631,664 I See Footnotes ( 6 ) ( 8 ) Class A Common Stock 11/07/2025 J( 9 ) 2,580,833 A 2,580,833 I See Footnotes ( 7 ) ( 8 )

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 Series B-2 Convertible Preferred Stock ( 2 ) 11/07/2025 C 640,727 ( 2 ) ( 2 ) Common Stock 640,727 ( 2 ) 0 I See Footnotes ( 3 ) ( 8 ) Series B-1 Convertible Preferred Stock ( 2 ) 11/07/2025 C 18,845 ( 2 ) ( 2 ) Common Stock 18,845 ( 2 ) 0 I See Footnotes ( 4 ) ( 8 ) Series C Convertible Preferred Stock ( 2 ) 11/07/2025 C 1,569,015 ( 2 ) ( 2 ) Common Stock 1,569,015 ( 2 ) 0 I See Footnotes ( 5 ) ( 8 ) Series D Convertible Preferred Stock ( 2 ) 11/07/2025 C 446,103 ( 2 ) ( 2 ) Common Stock 446,103 ( 2 ) 0 I See Footnotes ( 5 ) ( 8 ) Series A-1 Convertible Preferred Stock ( 2 ) 11/07/2025 C 20,000 ( 2 ) ( 2 ) Common Stock 20,000 ( 2 ) 0 I See Footnotes ( 6 ) ( 8 ) Series A-2 Convertible Preferred Stock ( 2 ) 11/07/2025 C 33,000 ( 2 ) ( 2 ) Common Stock 33,000 ( 2 ) 0 I See Footnotes ( 6 ) ( 8 ) Series B-1 Convertible Preferred Stock ( 2 ) 11/07/2025 C 2,261,377 ( 2 ) ( 2 ) Common Stock 2,261,377 ( 2 ) 0 I See Footnotes ( 6 ) ( 8 ) Series C Convertible Preferred Stock ( 2 ) 11/07/2025 C 137,288 ( 2 ) ( 2 ) Common Stock 137,288 ( 2 ) 0 I See Footnotes ( 6 ) ( 8 ) Series A-6 Convertible Preferred Stock ( 2 ) 11/07/2025 C 2,260,520 ( 2 ) ( 2 ) Common Stock 2,260,520 ( 2 ) 0 I See Footnotes ( 7 ) ( 8 ) Series B-2 Convertible Preferred Stock ( 2 ) 11/07/2025 C 320,363 ( 2 ) ( 2 ) Common Stock 320,363 ( 2 ) 0 I See Footnotes ( 7 ) ( 8 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Humming Bird Ventures Management NV SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Hummingbird Opportunity Fund II CommV SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Hummingbird Ventures III CommV SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Hummingbird Dragons CommV SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Hummingbird Collective CommV SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X HB&Q NV (HB&Q COMPARTMENT III) SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Van den Brande Barend SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Decoster Lukas SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C91210 X Signatures Hummingbird Ventures Management NV /s/ Lukas Decoster 49/9 CommV, represented by its permanent representative Lukas Decoster, Director 01/20/2026 Signature of Reporting Person Date /s/ Barend Van den Brande 01/20/2026 Signature of Reporting Person Date /s/ Lukas Decoster 01/20/2026 Signature of Reporting Person Date Hummingbird Opportunity Fund II CommV /s/ Firat Ileri By: Hummingbird Ventures Management NV, its Manager By: Firat Ileri, Permanent Representative 01/20/2026 Signature of Reporting Person Date Hummingbird Ventures III CommV /s/ Firat Ileri By: Hummingbird Ventures Management NV, its Manager By: Firat Ileri, Permanent Representative 01/20/2026 Signature of Reporting Person Date Hummingbird Dragons CommV /s/ Firat Ileri By: Hummingbird Ventures Management NV, its Manager By: Firat Ileri, Permanent Representative 01/20/2026 Signature of Reporting Person Date Hummingbird Collective CommV /s/ Firat Ileri By: Hummingbird Ventures Management NV, its Manager By: Firat Ileri, Permanent Representative 01/20/2026 Signature of Reporting Person Date HB&Q NV /s/ Firat Ileri By: Hummingbird Ventures Management NV, its Manager By: Firat Ileri, Permanent Representative 01/20/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The total represents shares received upon conversion of shares of Series A-1 Convertible Preferred Stock, Series A-2 Convertible Preferred Stock, Series A-6 Convertible Preferred Stock, Series B-1 Convertible Preferred Stock, Series B-2 Convertible Preferred Stock, Series C Convertible Preferred Stock and/or Series D Convertible Preferred Stock. ( 2 )The reported security converted into the Issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the Issuer's initial public offering of Class A common stock (the "Offering") without payment of further consideration. The shares have no expiration date. ( 3 )The reported security is directly held by HB&Q NV (HB&Q COMPARTMENT III). ( 4 )The reported security is directly held by Hummingbird Collective CommV. ( 5 )The reported security is directly held by Hummingbird Dragons CommV. ( 6 )The reported security is directly held by Hummingbird Opportunity Fund II CommV. ( 7 )The reported security is directly held by Hummingbird Ventures III CommV. ( 8 )Humming Bird Ventures Management NV is the managing member of Hummingbird Opportunity Fund II CommV, Hummingbird Ventures III CommV, Hummingbird Dragons CommV, Hummingbird Collective CommV, and HB&Q NV (HB&Q COMPARTMENT III) (the Hummingbird Funds) and, as a result, may be deemed to beneficially own shares held by the Hummingbird Funds. Mr. Ileri, Barend Van den Brande, and Lukas Decoster are partners of Hummingbird Ventures Management NV and may be deemed to have shared voting and investment power over the shares held by the Hummingbird Funds. The Reporting Persons disclaim beneficial ownership of these shares except to the extent of their pecuniary interests therein. ( 9 )Pursuant to a reclassification exempt under Rule 16b-7, each share of Common Stock was automatically reclassified into one share of Class A common stock immediately prior to the completion of the Offering.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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