See my question from another angle if it is so hard to understand the way I expressed it first: What Stugs and Jpz4 are for if Pz4s are enough to deal with Shermans/Cromwell/T34.
The OST P4 has a slight edge. It's not a counter.
It's the surrounding metagame that makes the P4 so favourable and the Sherman less so.
On the USF side, their AT gun is behind a tech fork, and their current meta commanders can skip weapon racks in favour of doctrinal AI. They've got a decent chance of going into T3 with a fairly lackluster AT presence. If you're at an AT disadvantage, answering a P4 with a Jackson is just a safer choice.
Ostheer, however, is playing against the light vehicle happy USF, so it'll very likely have a Pak or some PGrens out. It can quite safely go for a medium tank. And besides, what are its other options? StuGs are cheap AT support that work well as a second tank but are questionable as a first. Ostwinds are trash.
If you want to see more Shermans, rework the Stuart to be more like the AEC and Puma. Then LT builds can use it to support a Sherman against a P4.
If you want to see fewer P4s, buff the Ostwind. Give Ost a reason to want to build that over a P4. This'll also mean more Allied basic mediums, as the Ostwind
doesn't have an edge against them.