This is a really hard achievement!!! I recommend you play CONQUEST on BACKSTAB. First, pick the special op class and go to flag/base 4 or 5. They are good 4 close quarter combat since they're buildings. Camp with a knife inside one of the bases like in a corner or next to a entrance and try to get lucky or stealthy. After 4 kills you should get a in-game message saying medal obtained or something like that. Achievement pops up at the end of the game so don't quit. It's easier 2 get by boosting, just have the person getting the achievement go 2 the opposite main base and have the person getting killed camp in the main base waiting in something like a tent.
Update: Due to servers shutting down, online achievements can't be achieved.