For this achievement patience is key, you need be the sniper class while playing online. The key thing to remember is that you can not reload while taking all four shots and you can not die. Find a map that is open and a favorite of yours and begin sniping. You'll need to find a clever and hidden spot to camp at in medium range. Look for other people camping and/or follow others waiting for the time to strike. You need to get a headshot on four enemies without reloading. Not a hard feat but might require several attempts before you finally accomplish this task.