For this achievement you need to get 4 kills at once. Throwing a slime into 3 other enemies counts towards the achievement, since the one you throw dies as well.
Here is a quick clip of how I used this method on level 18, though I also saw another spot it could have been possible. (Level 13)
-(Level 25 may be easier for this achievement)-
*It's also worth noting that it is probably the easiest to do on levels that have a slime pool, that way the chompers can respawn. Though be careful because slimes will not respawn from the same pool*