The thing is with the P4 is it has very poor late game ability, when it gets out on to the field it's an excellent support tank till Jacksons come out and then your going to spend half the bloody game repairing the damn thing.
Specifically with Ostheer your problem is that your enemy has better infantry than you, and better mediums, so the answer to both of those things is Panther/Heavy tank. Why invest in something that is just going to get you mixed results? This of course, can be applied to Pgrens with shreks, if you all you going to get out of it is mediocre performance whats the point?
Choice you make. Do you want to get overun by allied Mediums well you try to stall for your overly cost effective heavy? Or do you want to use Mediums early to even the playing field? These are the choices that make RTS interesting.
Thats like saying I should never build Shermans because a Panther could arrive. The Sherman would get spanked by the P4 but its just cheaper and gives you a huge resource advantage if you can successfully save for the Tiger.
Your logic Flow seems like this to me:
Sherman counters infantry
Shreks counter sherman
P4s counter sherman
Jackson Counters P4
Shreks Counter Jackson (which is true and pretty ok)
Tiger counters all
???Counters Tiger...
Thats a weak argument. The games meta has DEVOLVED into the call in Meta. Its easier thus used. Doesnt mean this strat shouldnt have a downside that is punished heavily. I know you currently enjoy this with OKW but doesnt mean Ostheer should enjoy the same benefit. Or Soviets for that matter.
Not using a P4 doesnt make the P4 bad. Its your choice. If your infantry gets whiped by HE rounds that are designed to kill them Shrek or no well...shouldnt have stalled.