Capella IV was the inhabited fourth planet in the Capella system of the Alpha Quadrant. 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 2260s, this planet was located close to the Klingon-Federation border. Having rich deposits of topaline, Capella IV was valued by both the United Federation of Planets and the Klingon Empire. (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 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 is one of only two planets in The Original Series whose surface was depicted both on location (at Vasquez Rocks Natural Area Park) and on Desilu Stage 10. (Star Trek Encyclopedia (4th ed., vol. 1, p. 115))
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 Star Trek: Star Charts (pp. 33, 37 & 47) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Capella IV (Alpha Aurigae 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.
According to The Worlds of the Federation, the Capellans refer to their world as "Kohath".