Mercenam IV was a planet located somewhere in the space of the galaxy's Alpha or Beta Quadrants, the fourth planet in orbit of the Mercenam star system until the 23rd century.
History and specifics
In the early 2250s decade, the Mercenam system was devastated by a supernova. Captain Christopher Pike of the Federation starship USS Enterprise recalled hearing about the fourth planet's destruction upon a mention of a virus that had formerly been found on Mercenam IV. (TOS novel: Legacy)