The high speed of the Panther is off set by it's awful accuracy while on the move. Meaning you need to stop before each shot in order to reliably hit your target. Ostheer is also a defensive oriented faction meaning the high mobility of the Panther doesn't make sense when you will normally not be chasing your enemy.
That's a small price to pay when you have such a damaging and durable vehicle. Compared to it's counterparts, the M36 and Firefly, only the Comet can call itself superior, mostly due to being multi-role and still able to battle Panthers.
Non-Panther tanks wouldn't become invalid at all, the OKW PIV is way, way more popular than the OKW PV. Basically if given the choice of generalist or specialist most people will continue to pick generalist.
The OKW P4 is
much stronger than the Ostheer version, and is cheaper than the Panther. This is a lot of incentive for OKW to use it.
Ostheer T3 doesn't include rocket artillery. Perhaps the fix here might be moving the Pwerfer to T3?
I've advocated this for awhile now, actually.
Thing is, UKF, USF, Soviets, and OKW all have the ability in a match to access their entire unit tree but Ostheer for some reason is the exception. The whole "bonus tier" myth makes no sense because nothing in Ostheer T4 is actually more powerful than what other factions get over the period of a normal game anyway.
It's certainly much more powerful than USF T3 and Soviet T4, OKW and Brits can match it but it's risky, just like teching to Ost T4 is risky.
Everything in T4 is expensive, you won't be seeing any Panther or Sturmpanzer IV spam even if you make T4 cheaper to acquire.
Regardless Ostheer T3 is suffering heavily now because by the time it hits the field it's been countered to death.
It's expensive, but it snowballs. And it'd be better to fix T3 than to give them an easy T4 to dominate with.