The correct answer is: " the tetrarchy"
Diocletian was the Roman Emperor between years 284 and 305. During this reign, he managed to stabilize the empire and brough the so-called Crisis of the Third Century, to an end.
Diocleatian decided to appoint Maximian as co-emperor, so that he would rule the Eastern side of the Empire while Maximian would rule the Western one. Each of them appointed a junior co-emperor, who operated under their command, so it ended up becoming a "rule of four" or tetrarchy, and each ruled a quarter fraction of the empire.
