Gabrielle Union's Sydney is not returning for Bad Boys: Ride Or Die, and her absence has allowed the franchise to officially replace her. Union joined the action franchise 21 years ago with Bad Boys II, as she was introduced as the sister of Marcus (Martin Lawrence) and a new romantic interest for Mike (Will Smith). Syd played a vital role in the sequel thanks to her relationship with both lead characters and her job as an agent of the Drug Enforcement Administration. The appreciation for her largely left audiences wanting more of Gabrielle Union's character in future Bad Boys installments.
https://screenrant.com/bad-boys-4-gabrielle-union-syd-replacement/