I would also submit that,
Shorter gas tube also reduces the gas pulse timing so elevated pressures are not a huge issue.
Sight radius isn't an issue with the M-4 since the primary mode of sight is with optics now, not 'iron' sights.
Also, not all M-4 weapons have a 16" barrel, some special application rifles are shorter, depending on application...
M-4 is simply the military designation for the carbine version of the M-16 variant.
*Shorter than 16" in the civilian world is a violation of federal law with out proper BATF paperwork.