Exhibit 99.4

Joint Filing Agreement

In accordance with Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, the persons named below agree to the joint filing on behalf of each of them of a statement on Schedule 13D (including amendments thereto) with respect to the Class A common stock, par value $0.01 per share, of Syneos Health, Inc., a Delaware corporation, and further agree that this Joint Filing Agreement be included as an Exhibit to such joint filings. In evidence thereof, the undersigned, being duly authorized, have executed this Joint Filing Agreement this [21^st] day of September, 2020.

Date: September 21, 2020	Advent International Corporation
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Double Eagle Investor Holdings, L.P.
	By: Double Eagle GP, LLC, its General Partner
	By: Advent International Corporation, its Sole Member
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Double Eagle GP, LLC
	By: Advent International Corporation, its Sole Member
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Advent International GPE VII, LLC
	Advent International GPE VIII, LLC
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration

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Date: September 21, 2020	GPE VIII GP S.à r.l.
	GPE VIII GP Limited Partnership
	AP GPE VIII GP Limited Partnership
	By: Advent International GPE VIII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	GPE VII GP S.à r.l.
	GPE VII GP Limited Partnership
	By: Advent International GPE VII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Advent International GPE VIII Limited Partnership
	Advent International GPE VIII-B Limited Partnership
	Advent International GPE VIII-B-1 Limited Partnership
	Advent International GPE VIII-B-2 Limited Partnership
	Advent International GPE VIII-B-3 Limited Partnership
	Advent International GPE VIII-D Limited Partnership
	Advent International GPE VIII-F Limited Partnership
	Advent International GPE VIII-H Limited Partnership
	Advent International GPE VIII-I Limited Partnership
	Advent International GPE VIII-J Limited Partnership
	Advent International GPE VIII-C Limited Partnership
	By: GPE VIII GP S.à r.l., its General Partner
	By: Advent International GPE VIII, LLC, its Manager		/s/ Justin Nuccio
	By: Advent International Corporation, its Manager		Justin Nuccio, Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration

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Date: September 21, 2020	Advent International GPE VIII-A Limited Partnership
	Advent International GPE VIII-E Limited Partnership
	Advent International GPE VIII-G Limited Partnership
	Advent International GPE VIII-K Limited Partnership
	Advent International GPE VIII-L Limited Partnership
	By: GPE VIII GP Limited Partnership, its General Partner
	By: Advent International GPE VIII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Advent Partners GPE VIII Cayman Limited Partnership
	Advent Partners GPE VIII-B Cayman Limited Partnership
	Advent Partners GPE VIII Limited Partnership
	Advent Partners GPE VIII-A Limited Partnership
	Advent Partners GPE VIII-A Cayman Limited Partnership
	By: AP GPE VIII GP Limited Partnership, its General Partner
	By: Advent International GPE VIII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Advent International GPE VII Limited Partnership
	Advent International GPE VII-B Limited Partnership
	Advent International GPE VII-C Limited Partnership
	Advent International GPE VII-D Limited Partnership
	Advent International GPE VII-F Limited Partnership
	Advent International GPE VII-G Limited Partnership
	By: GPE VII GP S.à r.l., its General Partner
	By: Advent International GPE VII, LLC, its Manager		/s/ Justin Nuccio
	By: Advent International Corporation, its Manager		Justin Nuccio, Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration

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Date: September 21, 2020	Advent International GPE VII-A Limited Partnership
	Advent International GPE VII-E Limited Partnership
	Advent International GPE VII-H Limited Partnership
	By: GPE VII GP Limited Partnership, its General Partner
	By: Advent International GPE VII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration
Date: September 21, 2020	Advent Partners GPE VII Limited Partnership
	Advent Partners GPE VII 2014 Limited Partnership
	Advent Partners GPE VII-A Limited Partnership
	Advent Partners GPE VII-A 2014 Limited Partnership
	Advent Partners GPE VII Cayman Limited Partnership
	Advent Partners GPE VII 2014 Cayman Limited Partnership
	Advent Partners GPE VII-A Cayman Limited Partnership
	Advent Partners GPE VII-B Cayman Limited Partnership
	Advent Partners GPE VII-A 2014 Cayman Limited Partnership
	By: Advent International GPE VII, LLC, its General Partner
	By: Advent International Corporation, its Manager
	/s/	Michael Ristaino
	Name:	Michael Ristaino
	Title:	Vice President of Finance – Fund Administration

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