Well, Shermans came in a confusing variety of versions, but from a visual standpoint the two key changes that will distinguish it from the oob version are:
1. Welded rather than cast hull; this gives a more squared look. (This characteristic isn’t necessarily an improvement per se, but does lend a very different look than the highly rounded cast-hull look)
2. 76mm rather than 75mm gun (which involves both a longer barrel and a different turret, again giving a significantly different look for FMG to mold.)
The difference in capability between the 75mm and 76mm Sherman is very analogous to the difference between the short-barrelled and long-barrelled Panzer IV or between the T-34/76 and T-34/85. There were Shermans with even bigger guns, “Super-Shermans” if you will, but they were mostly a post-war tank and used mainly by allies and not the US itself because the US had already moved beyond the Sherman to the M-26… to the M-46… to the M-47 but by this point we’re deep into the Korean War.
There were also suspension changes, perhaps the biggest being the HVSS suspension, typified by the “-E8” variants referred to by the troops in a deliberate pun as the “Easy 8” both because of their smoother ride and “Easy” being the military shorthand for the letter “E.” And, of course, there were numerous engine variants, though this would hardly be visible in a game piece. In theory, I suppose, any given M4 Sherman can be found with any given combination of hull-type, gun-type, suspension-type and engine type, though of course in practice the ones that worked better gradually replaced the ones that didn’t work so well, so that the HVSS suspension models tended to have the later gun and engine versions since it was one of the last innovations to come into use… The “-A3” (or “-A1” or “-A2” or “A4” or “A6”) designation after the M4 in the terminology was mostly about engine type, rather than being a simple linear improvement, so an “M4A4” isn’t necessarily better than and “M3A3.” Probably the definitive war-time variant was the “M4A3E8(76)W,” which incorporated all of the improvements learned by war-time experience.