Based on this skin the fourth peeper is a cyborg peeper who was left behind when the other 3 evacuated, on a rare occasion, once you unite the 3 peepers, he can come down from above to check on them, but if you don't click fast enough, he'll fly away. When you get him he will leave the mech on the screen but pop out to wander with the other peepers. He will keep the cyborg eye to distinguish himself