The other problem for the M4 and the T-34 is German tank development passed both with the Tiger and the Panther and neither the USA or the USSR had a medium tank ready to go to match those tanks. Also the USA was working under a flawed doctrine at the time that tanks did the exploitation made by infantry while tank destroyers were supposed to fight the enemy tanks. Another factor in the continued use of the M4 was shipping and transport world wide of the tanks.