A small contribution:
From what I've played, 4B's balance seems to be pretty dang good! Certainly a lot better than 4A.
One thing though: It seems to me that Bubble Man should have an ammo bar maybe, because his bubbles are pretty dang strong. Or maybe their slight awkwardness is the drawback and he's fine. I'm just saying I tend to kill a lot with bubble lead.
I can't think of anything incredibly powerful or weak, though, so good job! We'll just have to wait for MM4 classes and beyond for more inevitable balance issues.