Sorry but I disagree. Back in the day T4 was the only Soviet tool to deal with tanks because T34 couldn't deal with anything else than a PzIV and this didn't change. Katyusha still works against infantry. Tested myself in recent game. It's not game braking unit anymore so o know it hurts.
You actuality repeated my argument yourself saying it's easier and better to go call ins than T4.
The old SU85, and by hence T4, was indeed the soviet meta long ago. But it was because SU85 was OP thanks to its acceleration that enabled fast attacks and retreats, making it (well microed) unflankeable.
Now SU85 post nerf is a clunky tank, that does a mediocre job as tank hunter because its speed doesn't match its lack of armour. Give it the frontal armor of a Jadgpanther or the Jackson's speed, but right now it is only useful in narrow maps, what is precisely the maps where ZiS does better, so both units overlap in their function being one of them cheaper.
And Katyusha kills infantry, indeed... but randomly. Some barrages wipes half enemy team, some others don't do anything at all. Nobody wants to gamble making an expensive and absolutely fragile unit that sometimes fails spectacularly. At least with Stuka and Pwerfer you now that everything under the barrage area will be seriously damaged. Katyusha is not a bad unit but it is very unreliable.
So, in T4 we have an useless unit (SU76), an unreliable one (Katyusha) and one which a T2 unit does approximately the same function (SU85). And all them are useless by themselves, any enemy tank or infantry with schrecks can destroy them.
It's not surprise that soviet players prefer T3 whose units, althought doesn't shine in anything, can defend by itselves and they make a great support for infantry and call-ins.
tl;dr
T4 is currently useless. Soviet players will continue using IS2 and ISU152 as heavy tanks because the main tank is T34 which can't do shit against the axis heavys.