ok i get it but what is your solution? lets say we will nerf its armour to make it paper thin like su85. Its performance vs TD's wont change as they have high pen anyway, it will still be OP with KT's/tigers and makes an underused unit even less desirable but at least it will be more venerable to med tanks.
From my experience, the problem does not stem from the unit itself but when its used in combination with other tanks. jp4 + KT or su85 with ISU152's both are almost impossible to overcome.
1 solution could be armour and VET nerf (give accuracy or more damage in VET instead or nothing at all) as well as a price decrease which could make it more appealing to go for (maybe have it med truck like before), but im curious about what you think should be done.
My initial thought would be to delete that +160 HP veterancy and reduce the ROF a bit, in exchange for a small price buff and making the stealth a bit more appealing (I personally don't like it although I recognize that it can be strong in some situations). I'd also move some of the reload veterancy to penetration. +10-20% (or split across two vet levels) probably would make it a bit more reliable while still not being OP vs heavies).
I think this could position the JP4 as a cheaper and less durable alternative to the Panther. Unfortunately this makes it also more of a SU85 copy.
Technically, it could also get the Jackson treatment and have thinner front armor so that mediums have better chances assaulting it.
Not sure if that is too much or maybe even not enough, but it would take away the indestructability.