While the first 3 Bunny Kill movies ended triumphantly, Bunny Kill 4 is more bittersweet, mainly because the potential love interest got killed by the Big Bad. On the plus side, Mottis has stated that the love interest's return is possible.
Tar Boy. A bunch of rich robot "fat cats" ruled the world and used robot slaves to work the land for their greed. Eventually they found an easier solution for mining resources and threw all the robot slaves in a boiling pool of tar. However, instead of dying, their memory chips infused with the tar and created a being made up of the tortured souls of thousands of robots. With every soul hungering for revenge, a battle erupts between Tar boy and the rich as he destroys and blasts the fat cats cohorts and robotic minions. But before Tar Boy could take revenge on the fat cats themselves, they uses a giant heater to melt him down, presumably killing him. However, it's revealed the old grandpa robot managed to collect Tar Boy's gooey remains years ago and believes that a piece of him escaped and is searching for the rest of his body. And when Tar Boy's body is complete, he will cast ruin on the fat cats once and for all.