Like most other players I use P4s to deal with allies infantry rather than with allies tanks since you have tank destroyers and At guns for that job.
And when thinking about the AI role the P4 is clearly over priced in that sector, being easily outperformed by sherman (due HE Shells) and by cromwells (due high speed crush). P4 vs Shermans vs Cromwell is rather a RNG fest, unless it's the P4 okw, so you could say they perform on the same level against each other.
I think it would be ok to decrease to costs of P4 to Sherman/Cromwell level, while reducing the P4 OKW version by 10 fuel, to show its superiority over Cromwell/Sherman.
34/76 vs pIV is also (or at least was for quite a while) an RNG fest that the 34/76 could win, despite costing 25 less fuel than a pIV and for some of that time having better AI. the pIV is certainly the better tank though.
point being that as long as relic doesn't have standardized prices for similar things it's really hard to judge cost efficiency of similar units between the factions.
It can't even penetrate the rear of an IS-2...it needs some pen buff.
IS-2 has 205 rear armour and the pIV has pens of 120/110/100. a pIV has at best a 59% and at worst a 48% chance of penetrating the rear armour of a IS-2. both of those values are greater than 3%, the cutoff value on penetration, and thus, at all ranges, the pIV can penetrate the rear armour of an IS-2.
i don't feel like doing the numbers but the same is true the frontal armour, with a lower pen chance.