Karl Four is a scientist of the reverse-time planet Arret in The Counter-Clock Incident, the son of Karla Five who looks far older than his mother and helps the Enterprise get home.
Karl Four appears in The Counter-Clock Incident, the last episode of The Animated Series. He lives on Arret, a world in a universe where time runs backwards, so that people are born old and grow younger until they become infants.
His mother Karla Five drags the Enterprise into this universe when her small ship passes through a nova. On Arret she takes Kirk and Spock to his laboratory. Karl Four appears to be an elderly man, while his own son is a baby in a playpen and his mother is a young woman.
He compares star charts with Spock and they work out that a nova must be burning in both universes at once to open a way back. He helps to ignite a dead star for the purpose, and the Enterprise returns under the command of Commodore Robert April as the crew becomes too young to run it.