The M4 is not a machine gun. Even if they get the LE/Military version, it is selective fire with the choices being semi auto or 3 round burst. Most LE models are semi auto only.
By law, the National Firearms Act, anything that fires more than one round per trigger pull is a machine gun. Besides, according to Colt's M4 web page, the M4 is "Available with a Safe/Semi/Full Auto 3-position selector (model R0977)". The US military just doesn't' buy them.
AVAILABLE MODELS
RO977 : Flat top, Safe/Semi/Full Auto
RO979 : Flat top, Safe/Semi/Burst
RO777 : Fixed handle, Safe/Semi/Full Auto
RO779 : Fixed handle, Safe/Semi/Burst
You are thinking of the M16A4 not the M4.