The Collaborative International Dictionary of English v.0.48:

Tetrarch \Te"trarch\, n. [L. tetrarches, Gr. ?, ?; te`tra- (see
   Tetra-) + ? a ruler, ? to lead; rule: cf. F. t['e]trarque.
   See Arch, a.] (Rom. Antiq.)
   A Roman governor of the fourth part of a province; hence, any
   subordinate or dependent prince; also, a petty king or
   sovereign.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tetrarch \Te"trarch\, a.
   Four. [Obs.] --Fuller.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

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).


Hitchcock's Bible Names Dictionary (late 1800's):

Tetrarch, governor of a fourth part