Socks Monsters exist on Sock Planet. Sock planet is where all the socks go when the worm-hole in the dryer opens up due to build up of static electricity. Sock planet is the reason you only get 1 stocking at Christmas.
Sock Monster #4 is a parasite. It takes the socks monsters and eats them thread by thread. It has a special digestive system to turn the fiber into the carbs and minerals it needs. This is the larval stage of SockMonster1.