4-LOM is a bounty hunter protocol droid that appeared only in video games, and has never been part of any official set.
4-LOM was assigned to work on the Kuari Princess where he became a translator to the ship's wealthy passangers. He found his work undemanding and after the Kuari Princess's computer altered his programming, he started to steal from the passengers. He then left the Kuari Princess forever and began a thieving life, and when Jabba the Hutt altered his programming further, he learned to kill and became a successful bounty hunter. Later, he even became partners with Zuckuss, and together they eventually joined the Rebel Alliance, rescuing rebels that had been stranded on Hoth after the Battle of Hoth.
He appeared in the Yoda Chronicles as one of the Bounty Hunters that Darth Vader sent after Luke Skywalker, the other Bounty Hunters included Cad Bane(sent to Endor), Dengar(sent to Corusant), Boba Fett(sent to Bespin), and Commander Gree disguised as a Wookie sent to Kashyyyk. 4-LOM is the one to find Luke on Tatooine, but in the movies it was Boba Fett that found Luke on Bespin.