if you're JP4 is missing alot then your RNG luck sucks like helpinghan's used to be. (ex. his p4 would never penetrate low health t34s and his shrecks would fail to hit a majority of the time.) What do you mean by heavy soviet units? is2? you shouldn't expect the jp4 to penetrate the is2 anyway, its like throwing a pak at a KT and thinking it will penetrate. the is2 has 375 armor and the KT has 400 armor. The JP4 has the same penetration as Jackson.
RNG gods hete me, but about JGPZIV I don't expect it to counter IS2 or anything, I just think that in most play styles it comes a bit late, and considering its fuel cost, low AI capabilities and being turret-less I think it needs some adjustments (like small fuel reduction).
On the other hand, as far as I know it has the smallest target size and it scales kinda well late game.
PS: This might be considered off-topic, but this OKW threads and stupid blobbing will stop only if there are viable AT tactics for the faction and their infantry is adjusted.