No one answer- depends on your wallet, your tastes, your uses. There are dozens of makers of good quality AR style rifles.
The M4 is capable of automatic fire, the AR 15 is not
Buy the best you can afford and like.
No.
The US Army presently uses the M4 carbine, a shorter and much modified version of the Armalite AR15.
AR15, M16, M4, Ruger Mini 14
AR-15 = Semi-Auto civilian rifle M4 = Select Fire Military rifle
Law Enforcement
Both are assembly line weapons. Stag in my opinion also has a better reputation. Unless you decide to go tactical, they are all pretty much the same. Bushmaster, dpms, stag ext..
You can buy a plastic m4 or you can buy a metal m4
m4 there are spare parts
18
Don't think so: laws here say you can't have EVEN semi-auto rifles. Let along full-autos.