If memory serves back in the day when T34 was in T3 it was really bad, penetration was worse and it didn't have the huge hull mg buffs so anti inf damage relied on lucky shots from the main gun. Best use was for ramming things.
The reason people went t3 was for the T70 mostly which was extremely potent squad wiping machine and the goal was to close out the game quickly before late game axis armor started to pile up. (or rely on call- in meta after t3) Also the T4 didn't require T3 and all the units T4 had were very situational/weak.
T3 had all the AI (HT, T70, T34), and T4 had the AT and arti. (SU-85, SU-76, Katyusha)
This made the SU-76 hilariously stupid to get, and the HT and T70 arriving typically too late compared to the usefulness of an actual piece of armor.
But the t70 could also crush for a period at this time, so that was a major advantage.
And as it was already stated, calling in the T34/85 battlegroup was almost always a better choice than going T4 for unturreted AT options.
Hence T4 wasn't really used for except for luxury rocket artillery. Kinda like Ostheer T4 nowadays (still.)
Current tech system and unit selection is way superior.