Tarsus IV was the site of a Federation colony in the 23rd century, the fourth planet of the Tarsus system. In 2246, an exotic fungus destroyed most of the colony's food supply, leaving the 8,000 colonists in serious danger of starvation. Governor Kodos made a drastic decision: he ordered the deaths of 4,000 colonists, so that the other 4,000 might live, selecting those to be killed according to his personal theories of eugenics. Earth forces arrived earlier than expected, but too late to save the 4,000 people Kodos murdered. All that was found of Kodos was a burned body, and he was presumed dead, until his chance discovery on Planet Q twenty years later.
Amongst the survivors of the Tarsus IV incident were James T. Kirk, Kevin Riley and Thomas Leighton, who were the only three living of nine survivors who knew Kodos' face, and would be able to identify him. (TOS: "The Conscience of the King")
Tarsus IV also appeared in the starchart Data and Picard were studying in Stellar Cartography in 2371. (Star Trek Generations)
Apocrypha
In the Shatnerverse novel Avenger, the famine is an act of eco-terrorism, the testbed for a plague created by the Symmetrists (Symmetrists at Memory Beta, the wiki for licensed Star Trek works), a radical environmentalist group who believed that the "cosmic plan" that formed the basis of the Prime Directive should apply to planetary ecologies as well - after the famine, they assisted Kodos in his escape and provided him with the name Anton Karidian.
In the Enterprise Logs short story "Though Hell Should Bar the Way", the USS Enterprise, under Robert April, was en route to Tarsus IV with relief supplies when it was attacked by a Klingon vessel commanded by Kor - though she was able to fight off the enemy ship (and dishonor Kor by not destroying it), they made it to Tarsus only to find that the massacre had already taken place, leaving April haunted, wondering if, had it not been for the Klingons, he might have reached the planet in time to save the colonists.