When I joined the USMC, the M16A1 was the service rifle.
I always had the feeling the lengthened stock on the M16A2 was an 'improvement' over the A1 on the square range, designed and approved by the match shooters, but not any advantage for combat. It is really too long especially with all the combat gear you are wearing.
The M4 or similar adjustable carbine stock is far better when you are in close quarters and wearing thick body armor, shoulder straps, etc.
I have never handled an A1. The stock is shorter than on a A2 or A4?
Yep. The A2 uses the same length extension tube, buffer and spring as the A1, but there is a spacer in the A2 stock behind the extension tube, to fill in the difference.
Never handled an A4, but I assume it's the same buttstock as the A2.
