There won't be a "4" in a CBS lineup in the future.
Jayson Tatum - assuming he fills out the next couple years - is what CBS wants in a forward. If/when he gets the roster he wants, he's only going to put one Big on the floor. The NBA may shift back to the days of 2 big men on the floor for everyone, but it won't any time soon.
A lineup of something like IT, Brown, Hayward, Tatum, Horford is the kind of thing Ainge and CBS are working towards.