Mortar - try to predict where he will run compared to how long it will take until the mortar impact. even if you only get 1 of them, you are pretty certain he will back away if not retreat.
Well..
By the time you reach t3 soviet player can at the same time reach t4. Meaning the pz4 would lose a lot of it's potential presence if moved to t4.
T-70 spam would be frequent and dominant because pak 40 is pretty lousy and stug isn't strong/fast enough to counter them as is. Attempting to catch it with shrecks will most likely end horribly bad.
By the time you finally do get a pz4 you'll presumably find at least 2 SU-85's waiting for you (provided you haven't been pushed back into your base)
TL;DR
I think it would horribly upset the balance around the t3 timer and make the pz4 completely void because at the time it could appear on the field, you would need something a lot stronger.
Really don't like that idea for numerous reasons. To be honest I highly doubt it will give the dynamic gameplay you are looking for and I would personally feel like being trolled for picking german every single game.