The rugged homeworld of the warrior Capellans, rich in the mineral topaline, contested by the Federation and the Klingons in Friday's Child.
Capella IV is the home of the Capellans, a tall, proud warrior people organised into the Ten Tribes and led by a Teer. The planet has deposits of topaline, a mineral needed for life-support systems on planetoid colonies, which makes its mining rights valuable.
In Friday's Child, Kirk, Spock and McCoy come to negotiate those rights and find a Klingon agent already there. A coup overthrows the Teer, forcing the officers to protect his pregnant widow in the hills until the Capellans accept the newborn heir.