You know that Ost stuff is better right? That kinda justifies the cost.
False. T3 is not necessarily any better than its allied counterparts. Also, late game for instance the t34 85 and easy 8s are some of the most versatile tanks in game, and the earlier su76 is now extremely powerful. The main thing in t3 is the stug, one of the only worthwhile tanks long-term. Ostwind is a joke in terms of its AI capability and panzer4 gets simply outclassed.
T4 is essentially non-viable except in large team games, rarely can a player afford to stall to get those units out, due to the fact that they will likely get overrun by allied tanks if they do. Prohibitive expenses + the time taken to overcome those expenses marginalizes t4 usage.