There's been a lot of debate lately about EA's decision to limit the classes in Battlefield 3 to only four classes. Many Battlefield veterans see a new Battlefield having five or as much as seven classes in the game. Mostly so that all bases are covered, medic, assault, spec-ops, anti-tank, and more. But with the way most FPS's have gone, there's always the options for load-outs and customize-ability.
Achievements and rewards won't just allow new guns as was the case with BF2, but different secondary, explosive, and primary choices can allow whole load outs that will tackle one or more static classes. A sniper with C4 could cover most bases of an engineer or spec-ops. A Spec-ops with an assault rifle and grenades could cover the bases of the assault class in Battlefield 3 as well.
We won't be able to see more Battlefield 3 weapons until we get a trailer on the 1st of March or see more screen shots. But we do know that the M4A1 and M249 will make an appearance in Battlefield 3 as weapons for probably Medic/Spec-ops or Support classes. Over the course of the next few months either people will realize that classes with a lot of customization will do the trick or DICE will reinstate the 5+ classes in Battlefield 3 that were standard in previous Battlefields.
