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.