The M4 carbine is a shortened variant of the M16 rifle, the standard-issue assault rifle of the United States military. Its smaller size and weight make it more favorable in confined spaces. Many variants exist in both military and civilian use.
The M4 carbine can be distinguished from the longer M16 rifle by the short handguard area toward the front of the weapon, and sometimes by the distinct notches cut into the barrel (example #2 above), which are necessary for attaching certain grenade launchers (example #3).
In order to avoid an excess of tags, please tag all M4/AR-15 variants with a short forend as m4_carbine.