What keeps the JP4 "bad" is not its performance, but its timing within the faction. Either you don't need it (P4 is better at fighting Allied mediums as well as makes up for more and more upgraded Allied infantry around the 15 min mark) or you don't want it later on because P4 and JP4 do not synergize as well.
Agree. Outside of the KT+JP4 combo the unit has no place in this faction. Put this unit into ost t4 instead of panther and you gonna feel how broken the unit can be.