Sec Form 4 Filing - Blackstone Energy Partners NQ L.P. @ Custom Truck One Source, Inc. - 2021-11-22

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 Blackstone Energy Partners NQ L.P. 2. Issuer Name and Ticker or Trading Symbol Custom Truck One Source, Inc. [ CTOS] 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) C/O CUSTOM TRUCK ONE SOURCE, INC., 7701 INDEPENDENCE AVENUE 3. Date of Earliest Transaction (MM/DD/YY) 11/22/2021 (Street) KANSAS CITY, MO64125 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 11/22/2021 S 2,549,735 D $ 8.2025( 1 ) 382,460 I See Footnotes( 2 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 3,149,642 D $ 8.2025( 1 ) 472,446 I See Footnotes( 3 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 305,733 D $ 8.2025( 1 ) 45,860 I See Footnotes( 4 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 156,378 D $ 8.2025( 1 ) 23,457 I See Footnotes( 5 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 3,036,582 D $ 8.2025( 1 ) 455,487 I See Footnotes( 6 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 3,109,809 D $ 8.2025( 1 ) 466,472 I See Footnotes( 7 )( 9 )( 10 )( 11 )( 12 )( 13 ) Common Stock 11/22/2021 S 15,096 D $ 8.2025( 1 ) 2,264 I See Footnotes( 8 )( 9 )( 10 )( 11 )( 12 )( 13 )

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 Blackstone Energy Partners NQ L.P. C/O CUSTOM TRUCK ONE SOURCE, INC. 7701 INDEPENDENCE AVENUE KANSAS CITY, MO64125 X BEP CTOS Holdings L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone Energy Family Investment Partnership SMD L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone Energy Family Investment Partnership NQ ESC L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone Capital Partners VI-NQ L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X BCP CTOS Holdings L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone Family Investment Partnership VI-NQ ESC L.P. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone Energy Management Associates NQ L.L.C. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Blackstone EMA-NQ L.L.C. C/O BLACKSTONE INC. 345 PARK AVENUE NEW YORK, NY10154 X Signatures BLACKSTONE ENERGY PARTNERS NQ L.P., By: Blackstone Energy Management Associates NQ L.L.C., its general partner, By: Blackstone EMA-NQ L.L.C., its managing member, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BEP CTOS HOLDINGS L.P., By: Blackstone Energy Management Associates NQ L.L.C., its general partner, By: Blackstone EMA-NQ L.L.C., its managing member, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BLACKSTONE ENERGY FAMILY INVESTMENT PARTNERSHIP SMD L.P., By: Blackstone Family GP L.L.C., its general partner, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Senior Managing Director 11/22/2021 Signature of Reporting Person Date BLACKSTONE ENERGY FAMILY INVESTMENT PARTNERSHIP NQ ESC L.P., By: BEP Side-by-Side GP NQ L.L.C., its general partner, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BLACKSTONE CAPITAL PARTNERS VI-NQ L.P., By: Blackstone Management Associates VI-NQ L.L.C., its general partner, By: BMA VI-NQ L.L.C., its managing member, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BCP CTOS HOLDINGS L.P., By: Blackstone Management Associates VI-NQ L.L.C., its general partner, By: BMA VI-NQ L.L.C., its managing member, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BLACK STONE FAMILY INVESTMENT PARTNERSHIP VI-NQ ESC L.P., By: BCP VI-NQ Side-by-Side GP L.L.C., its general partner, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BLACKSTONE ENERGY MANAGEMENT ASSOCIATES NQ L.L.C., By: Blackstone EMA-NQ L.L.C., its managing member, By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date BLACKSTONE EMA-NQ L.L.C., By: /s/ Tabea Hsi, Name: Tabea Hsi, Title: Authorized Person 11/22/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )This amount represents the $8.5000 public offering price per share of common stock, par value $0.0001 per share ("Common Stock"), of Custom Truck One Source, Inc. (the "Issuer"), less the underwriting discount of $0.02975 per share. The Underwriters have not yet exercised their Over-allotment Option to purchase additional shares of Common Stock. ( 2 )Reflects shares of Common Stock directly held by Blackstone Energy Partners NQ L.P. ("BEP NQ"). ( 3 )Reflects shares of Common Stock directly held by BEP CTOS Holdings L.P. ("BEP CTOS"). ( 4 )Reflects shares of Common Stock directly held by Blackstone Energy Family Investment Partnership SMD L.P. ("BE SMD"). ( 5 )Reflects shares of Common Stock directly held by Blackstone Energy Family Investment Partnership NQ ESC L.P. ("BE NQ"). ( 6 )Reflects shares of Common Stock directly held by Blackstone Capital Partners VI-NQ L.P. ("BCP VI-NQ"). ( 7 )Reflects shares of Common Stock directly held by BCP CTOS Holdings L.P. ("BCP CTOS"). ( 8 )Reflects shares of Common Stock directly held by Blackstone Family Investment Partnership VI-NQ ESC L.P. ("BE FIP"). ( 9 )Blackstone Energy Management Associates NQ L.L.C. ("BEMA") is the general partner of BEP NQ and BEP CTOS. Blackstone EMA-NQ L.L.C. ("BEMA NQ") is the managing member of BEMA. BEP Side-by-Side GP NQ L.L.C. ("BEP GP") is the general partner of BE NQ. Blackstone Management Associates VI-NQ L.L.C. ("BMA NQ") is the general partner of BCP VI-NQ and BCP CTOS. BMA VI-NQ L.L.C. ("BMA VI-NQ") is the managing member of BMA NQ. Blackstone Family GP L.L.C. ("BS GP") is the general partner of BE SMD. BS GP is wholly-owned by Blackstone's senior managing directors and controlled by its founder, Stephen A. Schwarzman. BCP VI-NQ Side-by-Side GP L.L.C. ("BCP GP") is the general partner of BE FIP. ( 10 )Blackstone Holdings III L.P. ("Holdings III") is the sole member of each of BEMA NQ, BEP GP and BMA VI-NQ. The general partner of Holdings III is Blackstone Holdings III GP L.P. ("Holdings III GP"). The general partner of Holdings III GP is Blackstone Holdings III GP Management L.L.C ("Holdings III GP Management"). Blackstone Holdings II L.P. ("Holdings II") is the sole member of BCP GP. Blackstone Holdings I/II GP L.L.C. ("Holdings I/II GP") is the general partner of Holdings II. Blackstone Inc. ("Blackstone") is the sole member of Holdings III GP Management and Holdings I/II GP. The sole holder of the Series II preferred stock of Blackstone is Blackstone Group Management L.L.C. ("BS Group Management"). BS Group Management is wholly-owned by Blackstone's senior managing directors and controlled by its founder, Stephen A. Schwarzman. ( 11 )Information with respect to each of the Reporting Persons is given solely by such Reporting Person, and no Reporting Person has responsibility for the accuracy or completeness of information supplied by another Reporting Person. ( 12 )Due to the limitations of the electronic filing system certain Reporting Persons are filing a separate Form 4. ( 13 )Each of the Reporting Persons (other than to the extent it directly holds securities reported herein) disclaims beneficial ownership of the securities held by the other Reporting Persons, except to the extent of such Reporting Person's pecuniary interest therein, and, pursuant to Rule 16a-1(a)(4) under the Securities Exchange Act of 1934 (the "Exchange Act"), each of the Reporting Persons (other than to the extent it directly holds securities reported herein) states that the inclusion of these securities in this report shall not be deemed an admission of beneficial ownership of all of the securities reported herein for purposes of Section 16 of the Exchange Act or for any other purpose or that the Reporting Persons are subject to Section 16 of the Exchange Act.

Remarks: Form 1 of 3

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