He's not big enough to reliably guard posts (which are almost exclusively 5s now), and he can't stay in front of perimeter players consistently. So you need to have him on the biggest perimeter oriented guy they have.

One thing I've noticed is you really seem to like your 4s to have post games, and that just doesn't mesh with most modern offenses. Almost all 4s now are taller 3 point shooters, just like Noah. Very few teams play with double posts.