"That is my 'now' name. RAMON-4 was rejected by my very makers." -RAMON-4
RAMON-4 was an entity residing within the gas giant Saturn who was pulling all the Rainbow Monkey dolls off of Earth and bringing them into orbit around the ringed planet.
When Numbuh 3 confronts the mysterious being at its core, RAMON-4 is revealed to be the remains of Rainbow Monkey 4ever, a living Rainbow Monkey doll created by the government in their own image, who were competing with the Russians to develop space-age technology (a real life event). Unfortunately, they succeeded. The monkey was hated by kids due to its dorky appearance, and was ejected into space. While drifting through space, Rainbow Monkey 4ever's plush body rotted away, and his metallic skeleton slowly started to connect with other pieces of floating junk it had collided with until they formed the vast robotic entity that it is today.
As punishment on the people who had rejected him, he developed sentience and began using his now godlike powers to take away all of Earth's Rainbow Monkeys. The name "RAMON-4" is a result of his metal plating, which displays the words "Rainbow Monkey 4ever", being rusted (In the shape of the number '3') to obscure most of the letters.
Upon hearing his tragic story, Numbuh 3 takes pity on RAMON-4, fulfilling his original purpose, to be loved by children. Now content with himself, RAMON-4 sends all the monkeys back to Earth and asks Numbuh 3 to stay with him forever. She refuses, to which RAMON-4 responds, "You can't blame a guy for trying." RAMON-4 is last seen singing the rainbow monkey theme song as the episode ends.