Barron is a former Safety.
Tree is a true 4-3 OLB.
Quinn is probably better at DE in a 4-3, well see on that one.
Barwin fits the position, he's just not as effective as he use to be.
Barron and Tree's fit at ILB have been discussed and discussed here tbh, so you're not alone there that's for sure. Still have hope for Tree, with Barron, well, we'll see, I remain very skeptical that he can do it but wouldn't be the first time I'm wrong.
4-3 DEs are typically a little heavier in the pants than Quinn. He should have zero problems playing in this defense as he has the wheels and everything else you want out there on the blindside. Height/weight-wise he's undoubtedly a better fit for Wade's system IMO. Bigger problem with Quinn is something along the lines of general give-a-F, or age, or whatever, because he's not the same guy he used to be. Dude looks like he's going to finish the season on the bench tbh.
Agree on Barwin, but as long as he logs most snaps at LDE he should be solid. He's a good fit there still and should still have some games where he flashes a bit.