The medium velocity 75mm gun on the M4A1,2,3 series of Shermans was effective against PzKwIV's. The PzIV wasn't well armored so in general a good side or rear shot with an M4 would take care of it. The PzIV's main gun (after the F2 of course) is still better than the main gun on the Sherman. I think it is better than the 76mm main gun on the upgunned Shermans as well. It would take the 90mm gun on the M36's or Pershings before the Americans had a better gun.
If we had a sherman I would want an M4A3E8(76)-the easy eight version or a British Firefly (with a 17lb AT gun) along with the basic 75mm gunned M4A3. I would also like to see a Cromwell, a T-34 (76mm and an 85mm upgraded one), a KV85 (counter for the Tiger).
I would like to see a Bren Carrier, a Sdkfz 251 halftrack, maybe a Marder or an Elephant (perk the daylights out of that), more trucks for resupply (I would actually do that sometimes).