Oceanus IV was the inhabited fourth planet in its star system.

In 2368, Starfleet added a diplomatic mission to Oceanus IV to the mission roster for the USS Enterprise-D. This addition cut down the time to explore the Phoenix Cluster from five weeks down to two. (TNG: "The Game")

This planet was only mentioned in dialogue.
According to Star Trek: Star Charts ("United Federation of Planets I"), Oceanus IV was listed as a Federation member in 2378.
The Star Trek Encyclopedia (4th ed., vol. 2, p. 99) identified Oceanus IV as a Federation planet.