The M4 is a good gun, but like Yar, I like the inertia systems used on the other guns. Which is why I just got my new M2.
The reason for the M4 for the military was that at times, when you start to load a bunch of accessories and weight onto the inertia guns, they can start to have failures. The inertia guns actually need a certain amount of recoil to function. Weight on the gun lessens that and can cause issues.
The gas system in the M4 is to allow for more reliable gun for the military after they start hanging a bunch of crap onto their guns.
If you dont need the military gun and are going to keep things simple, go with an M2 instead of the M4 and save quite a bit of money.