The XM177 and M16 Carbines led to the eventual development of the M4 Carbine.
Colt tried to determine the optimal shorter length carbine barrel and ultimately determined that (at the time) the shortest they could make the barrel without losing a considerable amount of reliability was 14.5" long.
You may also notice that the M4 barrel has a distinctive "notch" cut in the front of it. This is in order to properly attach the M203 grenade launcher.
Most M4's are also "light under the handguards", meaning that the barrel is skinnier under the handguards, similar to how the current M16 barrels are.
Here you can see an M4 profile barrel.
M4's have adjustable buttstocks like the early M16 carbines, but they are an updated version. M4's without rails have handguards that are "bigger" than most others and include dual heat shields.
It has many of the same features as the M16A4, including the flat top upper, A2 flash hider, pistol grip, and the same fire control group (Safe, Semi, Burst). M4A1's are full auto instead of 3-round Burst and have a thicker barrel profile (under the handguards).
M4's are currently being made and issued by Colt (who has the contract with the US Military) and are seeing more and more use over the M16's in the military, particularly in the US Army.