The military M16 is a 20 inch barrel fixed stock rifle. The M4 is 14.5" barrel collapsible stock carbine. There is absolutely no difference in the lower receivers or the upper receivers. I think the guy behind the counter was full of something that rhymes with spit.
"M4" is the military's designation for the current issue assault rifle. Any semi-auto equivalent is an ar15. Sometimes guys will call their 16" barreled collapsible stock rifles an M4 to distinguish it as set up like an M4 but this is technically incorrect.
Bighoss, I have to clear up a few things here. First of all the Army does not designate either the M16 or M4 as an assault rifle, never did. Second AR15 is a name that is patented to Colt and no one else is legally allowed to use it. People call their AR15 looking rifles ARs but if you will notice, none of them except Colt actually say AR anywhere on them. Bushmaster tried it and got sued by Colt and lost. Finally AR15 can be the civilian equivalent of either the M16 or the M4 but to get really technical about it when the AR15 first was marketed to the civilian world, there was no M4 invented yet. I don't think the A1 version had even come out yet. They were still to the best of my knowledge minus the chrome bore or forward assist.
Mforgery is a better nick name for the civilian Semi auto version of the M4.