RhymeZone: tetrarch definitions

Definitions of tetrarch:

noun: A governor or ruler of a quarter of a country, especially of a fourth part of a province in or client state of Ancient Rome.
noun: Each of the four members of a jointly ruling tetrarchy, notably of the Roman Empire since Diocletian.
noun: An officer in charge of a fourth part of a phalanx in Ancient Greece.

(Definitions from Wiktionary)