To get an easy M4 you must go to the level called 'motorcade interception' and get your weapon and go onto the spot where you are supposed to kill the man in the limo.The limo is surrounded by 2 jeeps with UN officers in it (briefing tells you to not kill them) ,but shoot the man in the limo when the first jeep enters the city.If you successfully kill him, the UN officers will climb out of the jeep and look for you, sneak up behind one of them and slit his throat with a kitchen knife and take his gun and clothes and escape. HAVE FUN!