Actually, there were many varients of the Sherman that had as good or better armor than the T-34 series or the Panzer IV series. It was up gunned and up armored in many different variations in the war, at least a dozen, if not more... just as the T-34 and Panzer IV were. All 3 were still in service in 1945.
It certainly was not a "heavy tank" and so any comparison to a Tiger is silly to start with, just as is any Sherman comparison to a Pershing or a JS-2.