Tetrarch

denotes "one of four rulers" (tetra, "four," arche, "rule"), properly, "the governor of the fourth part of a region;" hence, "a dependent princeling," or "any petty ruler" subordinate to kings or ethnarchs; in the NT, Herod Antipas, Mat 14:1, Luk 3:19, Luk 9:7, Act 13:1.

"to be a tetrarch," occurs in Luk 3:1 (thrice), of Herod Antipas, his brother Philip and Lysanias. Antipas and Philip each inherited a fourth part of his father's dominions. Inscriptions bear witness to the accuracy of Luke's details.