The military nomenclature is thus:
A1. Rear sights adjustable with a special tool and it was full auto.
A2. Much nicer rear sights with elevation and windage adjustable by hand. 3 shot burst.
A3. Full auto A2.
A4. Flattop, 3 shot burst.
Some manufacturers call the flattop A3, but that is not in keeping with military designations.
Personally, I prefer the A2. Although the scope mounts better on the A4, it is a pain in the ass to have to swap the carry handle in order to have the iron sights. While the A2 doesn't mount the scope as low, it is still quite adequate and you can use the iron sights with the scope attached. I have had an A4 and, while it was nice and looked more evil, I just prefer the A2.