Okay, I'm stumped by all the various numbers and designations for AR's.

I understand the original A1 with the carry handle and old sights.

The A2 has the new style sights.

A3?

A4?

The M-4 has the heavy barrel under the handguards and a step on the exposed barrel.

What are the other differences that I've missed and for some of the other models I've left out.

Just my 0.223 worth about ArmaLite's designations.

A2 = Forged "Carry-Handle/Sight uppers.

A4 = Picatinny "Flat top" rail uppers.

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.

A3 and A4 are the same basic thing from most manufacturers... it's a receiver with a rail unstead of the carry handle.  They'll use different designations for product recognition and trademark.

The barrel type has no bearing.  You can have an A1, A2, A3, or A4 with any barrel, from a 20" lightweight to a 14.5" M4-style to a 24" HBAR.

Quoted: The M-4 has the heavy barrel under the handguards and a step on the exposed barrel.

No, the opposite, M4 is skinny under the handguards and heavy with a step cut into it in front of the sight base.