tetrarch

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A subordinate ruler.
• noun One of four joint rulers.
• noun A governor of one of four divisions of a country or province, especially in the ancient Roman Empire.
• noun The commander of a subdivision of a phalanx in ancient Greece.

from The Century Dictionary.

• noun In the Roman empire, the ruler of the fourth part of a country or province in the East; a viceroy; a subordinate ruler.
• noun The commander of a subdivision of a Greek phalanx.
• Four principal or chief.
• In botany, having four centripetally developed xylem plates: said of some radial vascular cylinders.
• noun A stele which has four plerome strands.
• noun One of any group of rulers or chiefs.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Rom. Antiq.) A Roman governor of the fourth part of a province; hence, any subordinate or dependent prince; also, a petty king or sovereign.
• adjective obsolete Four.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a governor of part of a country, especially of a fourth part of a province in ancient Rome
• noun an officer in charge of a fourth part of a phalanx in ancient Greece