I disagree pretty strongly here. To Ph's point, we might need him out on the perimeter based on our lack of depth. However:
1/ The "4" as a position is effectively dead. This happened super quick, but pretty much every team plays 4 out these days, and the best ones (most of the NBA) are trying to play 5 out (tons of youtube videos talking about this). 4 is still used as a position designation, but not really practically different from any other wing position. Basketball went from 2 bigs --> "stretch 4" --> 4 out --> 5 out in a decade. Faster in the NBA but college is close. So "5 vs. 4" is really the same as "big man vs. wing" -- obviously oversimplified.
2/ Carr is way better as a big on offense. He's awesome in the PnR and a good finisher down low. His ability to shoot the 3 means we can play 5 out and clear the lane for our guards. If we try to use him as the roll-man when he's not the 5, it means you have to play the center on the perimeter. We saw when we tried to do that with Marsh, his dude was just sagging off crowding the paint. I suppose you can get by on offense this way if you have a 5 who can shoot like Keller. On the other hand, some of our worst plays are when Carr tries to create from the perimeter off the dribble -- he just can't do it. (I don't have any data on this, but he's way more comfortable with his back to the basket, but when starting at the perimeter he usually ends up way too far away and doing a bunch of pivots and pump fakes to no avail)
3/ Carr is way better defensively in the paint than on the perimeter. He's a good rim protector and shotblocker, but not a great perimeter defender. I would guess most of his fouls come when trailing a perimeter guy and trying to contest a shot from behind / the side. He isn't super great at 1v1 post defense which is probably the biggest weakness here, but it's not worse than when he tries to defend a quicker wing.
For any 247 subscribers, the data from the article I mentioned clearly backs all this up, imo.
tl;dr: Carr would have been a good "4" five to ten years ago, but the way teams are playing today he's much better on both ends at center. That said, he's versatile enough to play on the wing as well. Also I'm hopeful Reid will be a viable threat from 3 which would allow us to still use Carr as the main PnR guy without getting the lane too clogged.