Melnos IV was the fourth planet in its star system. This planet was known for its plasma geysers.

Sometime prior to 2369, Nella Daren led a team of geologists to study the plasma geysers here. To protect themselves from the heat, they cross-connected thermal deflector units, a method later successfully employed on Bersallis III. (TNG: "Lessons")

This planet was only mentioned in dialogue.
In the final draft of the script, this planet was identified as "Taurus IV". [1]