The Fourth Colony was a Voth colony.

In 2373, the Voth scientist Forra Gegen made arrangements with his supporters to meet him at this colony. He would be traveling with Chakotay to this colony, where the Human would be presented as proof of the Distant Origin Theory. Ultimatly, he was recalled to the Voth city ship before being able to do this, and without supporters he was unable to make his case effectively. (VOY: "Distant Origin")

This colony was only mentioned in dialogue.