Gamma Hydra IV was the uninhabited fourth planet orbiting Gamma Hydra. This planet was located in both the administrative area of Starbase 10 and the Gamma Hydra sector. It was a conventional M-class planet with an oxygen-nitrogen atmosphere and a normal mass.
In 2267, Gamma Hydra IV was irradiated by a rogue comet, killing all six colonists by an unknown radiation type which accelerated the aging of anyone on the surface. The leader of the colony was Robert Johnson, who was married to colonist Elaine Johnson.
The planet was visited for the annual examination of the science expedition by a landing party from the USS Enterprise, but four colonists were already dead and the Johnson couple was dying. Five of the six members of the landing party to Gamma Hydra IV were afflicted by the same radiation; all except one of those five could be, and subsequently were, healed from the effects. (The one of those five who could not be healed and was not healed, Lieutenant Arlene Galway, aged too rapidly and died too soon for the treatment formulated to be used on her.) (TOS: "The Deadly Years")