I don't think Foster is going to play the five. I think he sees himself as a stretch four. He's 7' but really doesn't want to do all of the dirty work.
I would agree from an offensive standpoint, Foster's skillset has been developed through his HS & AAU play as a stretch 4. That is a unique talent that TJ can leverage.
But can he play the 4 defensively? Whether they be small ball 4's, stretch 4's or traditional 4's. That's a tall order.
If Foster doesn't have the quickness to guard opposing 4's then he is going to have to be a 5 on the defensive end. He will have to do the dirty work as a 5 or that may limit his minutes.
That or TJ is going to need to play a lot of zone.
One frustration with Prohm was he talked a good game about his guys doing the dirty work (defense & rebounding), but he allowed his scorers to be one dimensional.
That may TJ's biggest cultural challenge- having players embrace the dirty work and have a bit of a mean/toughness streak