What does the Roman portrait sculpture The Four Tetrarchs represent?

It represents the Tetrarchs. This is a term which has been coined to indicate the emperors of the system of government created by the emperor Diocletian. In 285 he designated his fellow general Maximian as co-emperor. in 293 the designated two junior emperors (Caesars) who were subordinated to Diocletian and Maximian who became senior emperors (Augusti). This system had been termed the tetrarchy (rule by four in Greek). Hence the term tetrarchs for the four emperors.