On the M4 Carbine, Yes...it is removable.
There are two thumb screws on the left side, that you can loosen and it comes off the top rail for the handle.
The front sight can be removed too, but its more difficult, and you will need some sort of replacement to hold the handguard.
You probably want the M16 SPR....which already has the handle removed, and a flipdown front sight.
Bear in mind that they make 3 other AR style versions too.
M4 OPS, M16, M16SPR