I've completed level 3; the achievement shows on the main map, but level 4 isn't clickable, I can only replay level 3.
The only thing I can think of is that I killed the inhibitor before the leader.
I've tried playing level 3 all the way through again, killing the guy first, and the inhibitor second. Saved at the end and reloaded to do it again as well, and it still doesn't unlock.
Customer support service by UserEcho