Ostwind is a beast when stationary = defensive asset, you're not using it the same way as a P4 which is best used offensively. Now I agree on the fact, it isn't the first tank you build in 1vs1 as much as a Scott isn't the first tank you build in 1vs1 as USF.
stug = counter medium tanks
Pz4 = counter light tanks (stuart/AEC/T70) with a good anti-infantry potential and then with vet can go toes to toes with mediums
It is really important to understand what you are building and what for.
I gotta say, anything the Ostwind can do the p4 can do. Ostwind might be a little better against blobs, but it will almost never wipe squads, like the p4 can with some rng. For just a little more fuel you can get out the p4 and also have it counter tanks. It is just not worth it to get the Ostwind as Ostheer. It's the same thing with the scott, you are better off saving your time and fuel waiting for the extra fuel for the sherman.