If you're looking mainly to play CQB, then you could also look for an MP5, or similar platforms. The M4 is great, don't get me wrong, but its not your only option. However, this being said, customizing, as well as upgrading an M4 is great because of the sheer amount of parts available, as well as how easy it is to find parts for it.
I knew a man once who said "Death smiles at us all. All a man can do is smile back."