The term was understood in the Latin world as well, where Pliny the Elder glossed it as follows: “each is the equivalent of a kingdom, and also part of one” (regnorum instar singulae et in regna contribuuntur).
The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notably Sassanian Persia) and barbarians (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at the Rhine and Danube. These centres are known as the tetrarchic capitals.
The tetrarchs appeared identical in all official portraits. Coinage dating from the tetrarchic period depicts every emperor with identical features—only the inscriptions on the coins indicate which one of the four emperors is being shown. The Byzantine sculpture Portrait of the Four Tetrarchs shows the tetrarchs again with identical features and wearing the same military costume.
Source: Tetrarchy – Wikipedia