Some will classify the A3 being the standard style A2 but with the flat-top upper instead of the fixed carry handle.
The A4 or M4 style is one that looks like the M4 being that it has the relief cut into the barrel for the mounting of a grenade launcher. There will of course be differences between brands of rifles.
The good M4 clone should come with double heat shielded handgaurds, M4 style feed ramps, the M4 style 14.5" barrel, and a M4 style buttstock assembly, and I probably forgot something.