Boyd’s Bible Dictionary:
(fourth ruler). Originally, one governing the fourth part of a country. Under Roman rule, any ruler or petty prince of the republic and empire, especially in Syria (Matt. 14:1; Luke 3:1; 9:7; Acts 13:1). Sometimes called king (Matt. 14:9; Mark 6:14,22).
Concise Bible Dictionary:
Literally the governor over a fourth part of a province, but also applied to the governor of any small province. It is employed in the New Testament in reference to Herod Antipas, tetrarch of Galilee and Perxa; Philip, tetrarch of Ituraea and Trachonitis; and Lysanias, tetrarch of Abilene (Matt. 14:1; Luke 3:1,19; Luke 9:7; Acts 13:1).
Jackson’s Dictionary of Scripture Proper Names:
ruler of a fourth part (of a country)