sloped 45mm is not better than flat 80.
Only the later PIV's had 80mm on the front, the early and mid-war models had 50mm. This is modeled in game by the armor bonus Panzers get at vet 2.
45mm of armor at the T-34 60 degrees slope = 77mm effective armor. So with the later model PIV's it's pretty much equal on the front, with the T-34 having significantly better armor on its sides and rear.
But this is Company of Heroes, therefore Krupp Steel, so T-34s have notably worse armor all around even at vet 0, and the gap only widens at vet 2.
I think both Panzer 4s are fine, I'd like to see the T-34 be able to deflect shots every once in a while though