It's smaller, lighter and better suited for modern battles. And it might be
headed into the hands of U.S. Marines.
Marine Corps officials wrapped up testing two new rifles as a possible
replacement to the M-16A2 in stock now: the short M-4 carbine and the
M-16A4, an upgraded model of the rifle Marines use now.

http://www.estripes.com/article.asp?...4&article=9787