a system by which four men rule the empire. (p. 179)

Government by four rulers governing four different areas of the same state at the same time

system of Roman government implemented by Diocletian in A.D. 293 that distributed imperial power among four rulers or tetrarchs, two Augusti and two Caesares.

Tetrarchy (Greek: "leadership of four people") can be applied to any system of government where power is divided between four individuals but is rarely used. The most famous Tetrarchy is that instituted by Roman Emperor Diocletian in 293 and lasted until c. 313. The establishment of the Tetrarchy usually marks the resolution of the Crisis of the Third Century and the recovery of the Roman Empire.