Not true. M4's come in three different versions; semi-auto only, semi-auto and full auto, semi-auto and three shot burst. The mode of fire is determined by a selector switch. The semi-auto only version is completely legal to own by federal law, although several states have outlawed them because they fit into a home grown definition of "Assault Rifle".
The primary difference between an AR-15 and M4 is the barrel length and fixed butt stock versus a collapsible butt stock. AR-15's usually are equipted with a 20 inch barrel whereas the civilian M4's typically have 16 inch barrels.
Every part on AR-15's and M4's are interchangeable. Either can be built with many different barrel lengths and buttstock configurations.
Thanks so much for the good info.
Apparently the NYT has reported that the M4 carbine they were in possession of was a fully automatic version and is illegal in the state of MA