It's very niche.
The core concept here is a sidegrade that shifts the JPIV from a high damage, low pen tank destroyer into a lower damage, higher pen tank destroyer. I've proposed numbers, but you could always tweak them.
You could make similar refit upgrades for other units that only work well in one mode.
Imo the only issue with the JP4 is its place in tech. If it's along side a panther you will 9/10 times take the panther due to its mobility traits.
The JP4 could work in HQ after 2 trucks to allow okw something that isn't God tier but still good as AT if the schwere goes boom. It also allows an okw player to go med+mech and still have a decent TD to help fight TDs.
It's not a spearhead unit after all, it's primarily defensive in a very aggressive faction which is why it feels so out of place.