Why is the JP4 too expensive to counter medium tanks? It costs roughly the same as a Su85 and less than Jackson and Firefly. It's also next to impossible for Allied medium tanks to do much damage to it because of 230 frontal armor. Unlike allied TDs that all get penned at a high percentage by P4s
The JP4 isn't bad, it's redundant.
You want a quick counter to enemy mediums? You get the OKW P4. It'll beat them and is your best anti-infantry medium. You want a tank-killer, you almost always spend the extra 45 FU on a Panther, which has AI capability and will never become outclassed by the deployment of enemy heavies.
The JP4's only really good against enemy TD spam, because it matches range with them and cloaks.