Originally Posted by JonM
its just a model name. a true m4 carbine is select fire with a 14.5" barrel as its two most distinguishing features.
colt has marked a couple of models of their civilian and law guns with 16" barrels and semi only with M4 for a while. its still an ar15.
tenntyrant, JonM's answer is dead on - as it is you have an AR-15 modeled after the military M4. Your Colt is very, very close to being a true select fire M4 and if it were legal
you could get it even closer by shortening the barrel to 14.5" and buying a select fire lower receiver. Of course it would be insane to consider such a thing, just pointing out differences.
Often AR-15s modeled after the M4 are called "M4geries" to distinguish between the real thing. Close, but Uncle Sam won't give you the cigar.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek