To be clear...you can get this anytime there's a hoarde of 4 or more zombies attacking you, but it is a special melee move. No matter what weapon you have equipped, when a zombie grabs you they will be trying to push in and bite you. A cursor appears (looks like a small circle with a pointer). You have to use the right stick to position that cursor over the zombies head/face and then pull right trigger to knife them in the head.
So make some noise to attract a group of zombies and then let the 1st one grapple you. After you successfully do the first kill, it automatically selects the next zombie to repeat the event. Four in a row without being killed and the achievement pops.