Whatever you upgrade it at tier 3 or tier 4 both will slow down your tank.
One can get upgrade from around minute 6 and enjoy more cost efficient infatry but delay his vehicles or one can rush vehicles and unlock it after T4 unit. It is a choice.
These may affect a lot in 1vs1 match while you need to engage with panzer IV that currently arrived time is 12 minute.
A T-34 is is not a great counter to a PzIV while soviet already got their other counter improved like PTRS Penals and SU-76.