If you want Werfer on T3, something has to go, it doesn't make sense for the StuG-G or the PZ4 to be in T4, so Ostwind is the best candidate.
Why can't we put P4 in T4? T3 would be pure support units which will fit the OH theme. With P4 in T4 we can now drop T3 price since rushing T3 wouldn't break the game now and OH can now get a Pzwerfer before Soviet get a T70 out or at least at the same time.
With both Pz IV and Panthers in T4 I think Panther can have a price drop now so OH player have to choose either to rush out a T4 or wait for Panther which would be all better at a slight price increase. Panther just needs to fix its accuracy and its price and it will be perfect.