Capella IV was the inhabited fourth planet orbiting the primary Capella. The planet was located close to the Klingon-Federation border. It was the homeworld of the Capellans, a pre-warp humanoid species, as well as the Capellan flower and the Capellan power-cat.
In the mid-23rd century, Capella IV was valued by both the Federation and the Klingons for its rich deposits of topaline. (TOS: "Friday's Child"; TAS: "How Sharper Than a Serpent's Tooth", "The Counter-Clock Incident")
In 2267, the planet served as a pawn in the Federation-Klingon Cold War, during a trade dispute between the Klingons and the Federation over topaline mining rights. The crew of the USS Enterprise was able to secure the rights for the Federation. (TOS: "Friday's Child")
In 2293, the location of Capella in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
In the first draft of the script (dated 20 April 1967), Capella IV was called "Ceres VII", and the Capellans were called the "Cereans". The script described the planet as "a small planet that is predominantly greenish-blue, with red seas."
Capella IV was given a "face lift" for the 2006 remastered version of "Friday's Child", with a digital planet replacing the stock footage from the classic episode.
According to StarTrek.com and the Star Trek Encyclopedia (3rd ed., p. 63), Capella IV was an M-class world.  According to The Worlds of the Federation, the Capellans refer to their world as "Kohath".
According to Star Trek: Star Charts (pp. 33, 37 & 47), Capella IV (Alpha Aurigae IV) was located in or near Federation space. Capella IV was ruled by the Ten Tribes of Capella, and was established as a protectorate of the Federation in 2267. The dominant species were the humanoid Capellans. In 2378, there were an estimated 160,000 Capellans living on the planet. A major attraction of the planet was the tomb of Leonard James Akaar.