is the weapon that the bandits use that looks like a m4/ar 15 useable by the PC.

I would be surprised if there wasn't some sort of m4 clone in the game its pretty standard as far as weapons go.

No.....Bandit weapons will disappear when they drop it. You can loot the bodies for ammo though, but not weapons.

I believe we get that weapon anyways, don't we?

I'd assume we'd get acess to enemy weapons at some point, wouldn't make much sense to create a seperate set of weapons just for npc's.

Every id game has all weapons in game available for the player so you will beable to have it, not to mention that in dozens of interviews they have said that theres alot of weapons and items and that everything you pick up (even wepons) can be kept or sold (obviously your not gonna sell a weapon but you get my point). P.S. They have said multiple times that its going back to id's roots which includes being able to hold all weapons in game.