I'd say make the M4 one of those unlock-able weapons, like the silenced MP9, except it isn't really silenced like the Glock, it should be still very loud and has a distinct noise.

And Mid tier unlock, a few levels maybe after the MP9 or trade it for one of the worse weapons as most unlock-able guns are good until the sniper pistol and up.

I'd say make the M4 one of those unlock-able weapons, like the silenced MP9, except it isn't really silenced like the Glock, it should be still very loud and has a distinct noise. And Mid tier unlock, a few levels maybe after the MP9 or trade it for one of the worse weapons as most unlock-able guns are good until the sniper pistol and up.