I went from the M4, to the SCAR from level 20-40 or so. Now onto the M16.
People tell you to use it as much as possible because you get 10,000xp once you master a gun.
But in my opinion, you can get 10,000 xp from 1-2 games of multiplayer. So just use the gun that you get the most kills with.
SCAR is much more powerful than M4, allows you to not use the stopping power perk and use something else.