The question for me is not so much athleticism but rather will Johnson be a better defender and shooter at the 4 position.
I mean, he has the physical strength to do it. I'm not saying he can't play spot 4 minutes, he might be able to, and do a very good job, but it will require him to be a very intense and physical defender. I mean, Draymond Green really gives Randolph and Gasol trouble in post-ups. He is 4'' shorter in standing reach though, which is a pretty big difference.
I think most scouts, gms, and fans thought he was much bigger though, I know I did.