The word Tetrarchy means "rule of four." It derives from the Greek words for four (tetra-) and rule (arch-). In practice, the word refers to the division of an organization or government into four parts, with a different person ruling each part. There have been several Tetrarchies over the centuries, but the phrase is usually used to refer to the division of the Roman Empire into a western and eastern empire, with subordinate divisions within the western and eastern empires.
The Roman Tetrarchy
Tetrarchy refers to the establishment by the Roman Emperor Diocletian of a 4-part division of the empire. Diocletian understood that the huge Roman Empire could be (and often was) taken over by any general who chose to assassinate the emperor. This, of course, caused significant political upheaval; it was virtually impossible to unite the empire.
The reforms of Diocletian came after a period when many emperors had been assassinated. This earlier period is referred to as chaotic and the reforms were meant to remedy the political difficulties that the Roman Empire faced.
Diocletian's solution to the problem was to create multiple leaders, or Tetrarchs, located in multiple locations. Each would have significant power. Thus, the death of one of the Tetrarchs would not mean a change in governance. This new approach, in theory, would lower the risk of assassination and, at the same time, made it nearly impossible to overthrow the entire Empire at a single blow.
When he split up the leadership of the Roman Empire in 286, Diocletian continued to rule in the East. He made Maximian his equal and co-emperor in the west. They were each called Augustus which signified that they were emperors.
In 293, the two emperors decide to name additional leaders who could take over for them in the case of their deaths. Subordinate to the emperors were the two Caesars: Galerius, in the east, and Constantius in the west. An Augustus was always emperor; sometimes the Caesars were also referred to as emperors.
This method of creating emperors and their successors bypassed the need for approval of emperors by the Senate and blocked the power of the military to elevate their popular generals to the purple. Source: "The City of Rome in late imperial ideology: The Tetrarchs, Maxentius, and Constantine," by Olivier Hekster, from Mediterraneo Antico 1999.
The Roman Tetrarchy functioned well during Diocletian's life, and he and Maximian did indeed turn over leadership to the two subordinate Caesars, Galerius and Constantius. These two, in turn, named two new Caesars: Severus and Maximinus Daia. The untimely death of Constantius, however, led to political warring. By 313, the Tetrarchy was no longer functional, and, in 324, Constantine became sole Emperor of Rome.
While the Roman Tetrarchy is the most famous, other four-person ruling groups have existed through history. Among the best-known was The Herodian Tetrarchy, also called the Tetrarchy of Judea. This group, formed after the death of Herod the Great in 4 BCE, included Herod's sons.