[Form 4/A] Hyliion Holdings Corp. Amended Insider Trading Activity

Rhea-AI Filing Summary

Hyliion Holdings Corp. insider Joshua T. Mook, the company’s Chief Technology Officer, filed an amended Form 4 to correct a previously reported tax-withholding transaction. The earlier filing had shown shares of common stock withheld on August 25, 2025 under transaction code "F" due to an administrative error by the issuer’s third party service provider. The amendment clarifies that no shares were actually withheld on that date and updates Column 5 of Table I. Following the corrected entry, Mook is reported as beneficially owning 529,479 shares of common stock, held directly.

SEC Form 4

FORM 4

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
or Section 30(h) of the Investment Company Act of 1940

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

1. Name and Address of Reporting Person^*

MOOK JOSHUA T.

(Last) (First) (Middle)

C/O HYLIION HOLDING CORP.

1202 BMC DRIVE, SUITE 100

(Street)

CEDAR PARK TX 78613

(City) (State) (Zip)

2. Issuer Name and Ticker or Trading Symbol
Hyliion Holdings Corp. [ HYLN ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

	Director		10% Owner
X	Officer (give title below)		Other (specify below)
	Chief Technology Officer

3. Date of Earliest Transaction (Month/Day/Year)
08/25/2025

4. If Amendment, Date of Original Filed (Month/Day/Year)
08/25/2025

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	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	08/25/2025		F(1)		0	D	$0	529,479	D

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 (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	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 (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (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: 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

Explanation of Responses:

1. This Amended Form 4 has been filed to delete a reference to shares withheld to satisfy taxes on the reporting person's Form 4 filed with the SEC on August 25, 2025. The shares were initially reported as withheld as a result of an administrative error by the issuer's third party service provider. No shares were withheld on August 25, 2025 and Column 5 of Table 1 has been revised accordingly.

Remarks:

	/s/ Joshua T. Mook	11/17/2025
	** Signature of Reporting Person	Date
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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.

FAQ

What does the Hyliion (HYLN) Form 4/A filing report?

The Form 4/A reports that Chief Technology Officer Joshua T. Mook amended a prior insider trading report to correct an error related to a tax-withholding transaction dated August 25, 2025.

Why did Hyliion (HYLN) file an amended Form 4 for its CTO?

The amendment was filed to delete a reference to shares withheld to satisfy taxes that had been reported on August 25, 2025. The filing states those shares were reported due to an administrative error by the issuer’s third party service provider.

Were any Hyliion (HYLN) shares actually withheld for taxes on August 25, 2025?

No. The amended filing states that no shares were withheld on August 25, 2025, and the prior reference to withheld shares has been removed.

How many Hyliion (HYLN) shares does the CTO own after the corrected transaction?

After the correction, Chief Technology Officer Joshua T. Mook is reported as beneficially owning 529,479 shares of Hyliion common stock, held directly.

What was changed in Table I of the Hyliion (HYLN) Form 4/A?

Column 5 of Table I was revised to reflect that no shares were withheld on August 25, 2025, while showing a post-transaction direct beneficial ownership amount of 529,479 shares.

Who signed the amended Hyliion (HYLN) Form 4/A?

The amended Form 4/A was electronically signed by /s/ Joshua T. Mook, the reporting person and Chief Technology Officer.