The position that someone plays determines if they are a 4 or a 5. Think about Michigan’s base two-guard offense ,it starts something like this.
You have your two-guards (hence the name) at the top, and each the ‘3’ plays the left wing and the ‘4’ plays the right wing. The 5 is in the paint and the primary screener.
To play the ‘4’ at Michigan, you have to be able to play on the wing like DJ and Zak are doing in this spot. Right now, Wagner has rarely if ever done that which is why he should be looked at as a ‘5’. I can’t really think of anyone who has started primarily as a 5 and eventually played the 4 under Beilein. There was a lot of talk about it with Mitch and I think they tried it for a couple of games, but I don’t remember it working or sticking before his injury.
As far as Donnal being a big and not Wilson… Donnal played the 5 for his whole career, Wilson played the 4 for 80% of his career and then shifted to the 5 because it gave Michigan a new look.