Tetrarch

Strictly the ruler over the fourth part of a province; but the word denotes a ruler of a province generally (Matt. 14:1; Luke "3:1, 19; 9:7; Acts 13:1). Herod and Phasael, the sons of" "Antipater, were the first tetrarchs in Palestine. Herod the" tetrarch had the title of king (Matt. 14:9).

Definition of Tetrarch:
"governor of a fourth part"