Belarius IV was a planet, the fourth planet in the Belarius system, and was located in Federation space somewhere in the galaxy's Alpha or Beta Quadrants.
History and specifics
In 2270, the USS Enterprise orbited Belarius IV to beam up Number One, then left to rendezvous with the USS Yorktown so that she could assume command of that starship. En route, however, the Yorktown disappeared. During its investigation, the Enterprise encountered a Colonizer ship. (TOS - New Visions comic: "A Scent of Ghosts")