A hinted story beat in Gen V season 2's trailer seems to suggest that The Boys has a new Billy Butcher replacement ahead of the show's final installment. Towards the end of The Boys season 4, Billy Butcher reached the end of his wits and took some extreme measures to ensure that things went his way.
His actions in The Boys season 4's ending arc suggested that he would no longer team up with Hughie and the crew and go his own way to fulfill his mission against Homelander and all other supes. Since Billy Butcher is seemingly adopting a completely different role moving forward, The Boys will have to find a way to fill his absence in the titular team.
Interestingly, Gen V season 2's trailer hints that the television franchise has already found his worthy replacement