The Roman Empire was for a time after 293 ruled by a tetrarchy (a group of four rulers), instituted by Emperor Diocletian. The tetrarchy consisted of two Augusti (senior emperors) and two Caesars (junior emperors). The empire was territorially divided into western and eastern halves, with a senior and a junior emperor in each half.
The Portrait of the Four Tetrarchs symbolizes the concept of the tetrarchy, rather than providing four personal portraits. Each tetrarch looks the same, without any individualized characteristics, except that two, probably representing the older Augusti, have beards, and two do not who might have symbolized the Caesars. The group is divided into pairs, each embracing, which unites Augusti and Caesars together. The overall effect suggests unity and stability.
(from wikipedia – for full article: https://en.wikipedia.org/wiki/Portrait_of_the_Four_Tetrarchs)