It is true that the T-34 and the M4 are quite similar in effectiveness. However, the real difference between them is not in the quality of the vehicle, but in timing and its impact on the battlefield. When the T-34 first saw combat in the summer of 1941 is was undoubtedly the best tank in the world. It was a shock to the Germans who had very little they could counter it with. When the M4 entered combat in late 1942 the Germans had learned from more than a year of the experience fighting the T-34 on the eastern front and developed countermeasures against it. They had upgunned and uparmored their PzIII, PzIV and StuG III to counter the T-34, and developed anti-tank tactics using FlaK 88s and the new 75 mm PaK 40. There were even Tigers in service when the M4 showed up. So while the T-34 was "Gott im Himmel!" when it arrived, the M4 was "meh". Sort of like if the Spit I had showed up in 1941...