You dont seem to understand the roles of the different tanks. A Jackson does not have to go toe to toe with a Stug, since its not made to counter a stug, but rather enemy heavy tanks (or mediums like the p4). The Sherman is supposed to have a stronger AI role, with which you can exert ernormous amounts of pressure on your opponent. The P4 on the other hand doesnt even have a refined role, its just built because OST has not many viable options...
jackson can really really easy flank a stug and jackson wins for 1 second reload and with the sherman 76 mm gun they can counter OKW p4 so yeah try sherman 75 mm gun cant even pen mid range weh p4 the sherman 76 not only beats OKW p4 at mid range but with half health