In 2246, an exotic fungus destroyed most of the colony's food supply, leaving the eight thousand colonists in serious danger of starvation. Governor Kodos made a drastic decision: he ordered the deaths of four thousand colonists, so that the other four thousand 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 four thousand 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.
Among the survivors of the Tarsus IV incident were James T. Kirk, Kevin Riley, and Thomas Leighton, who were the only three of nine survivors who knew Kodos' face, and who would be able to identify him. By stardate 2817, they were the only three of the original nine witnesses still alive. (TOS: "The Conscience of the King")
Presumably, Tarsus IV was located in the Alpha Quadrant.
According to Mike Sussman, a computer readout on the USS Defiant in "In a Mirror, Darkly, Part II" stated that Hoshi Sato was among those killed by Kodos on Tarsus Four. However, the relevant section of the bio did not appear in the finished cut of the episode.
In the Shatnerverse novel Avenger, the famine was an act of eco-terrorism, the testbed for a plague created by the Symmetrists (Symmetrists (β)), 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.