Sander explained to you plenty of times its effectiveness vs mediums.
If you didn't understood that, you aren't going to understant it when I'll do it as well, so no need to waste a breath.
So you can't, it's ok.
JP is only slightly better than SU-85 vs mediums yet it more expensive. So that brings down you theory that SU-85 and SU-76 are "different TDs for different targets". The SU-85 can counter mediums just fine.
Existence of panther is extremely relevant, because panther to JP4 relation is exact the same as SU-85 to SU-76 relation - different TDs for different targets. I don't see you complaining about SUs differences and how 76 doesn't have 200+ pen, I can't see why you have a problem with JP4.
You want JP4 to perform like allied end tech TD, you'd have to make it OKW end tech TD, that means stripping panther of its AT role or completely removing it.
Great lets lower the price of the JP to 100 fuel so it role is to counter mediums. After all stug SU-76 and M10 that according to theory are meant to counter only mediums are allot cheaper.
Not really, already is JP4 an end tech OKW that already cost more than SU-85. It simply does not perform as good. And it can easily coexist with Panther. One could be a dedicated long range TD with a similar role to SU-85 and the other brawler.