The Four Tetrarchs
Portrait of the Four Tetrarchs, which is embedded into the wall connecting the Church of San Marco and the Palazzo Ducale, is one of the many spolia to be seen in Venice. Commonly confused as Byzantine, the porphyry stone Tetrarchs are from the last phase of Polytheistic Roman culture, which is dated back to 305 CE. After Emperor Alexander Severus was assassinated in 235 CE, Diocletian decided to divide power into a tetrarchy: two senior emperors (Augusti) and two junior emperors (Caesars). The Four Tetrarchs that are seen at the basilica are assumed to be Diocletian, Maximianus, Galerius and Constantius. The Tetrarchs are not individualized. With that being said, the possible reason was to make it relevant for future emperors to come as well as showing the equality of their union.
The only thing depicting the difference between the Augusti and the Caesars is that the Augusti have beards while the Caesars have no facial hair.
The use of the porphyry stone for the Portrait of the Four Tetrarchs was not done by accident. This tough red Egyptian stone, composed of large crystals was used most likely because of the color and its use on imperial sculptural objects. When seen in the light, the porphyry stone gives off a purple tone, which was associated with royalty.
The Four Tetrarchs are seen at eye level and are quite large. The look in the eyes of the Tetrarchs give a sense of fear to the viewer, as if you are invading their area. Being placed at the cornerstone of the church that is right beside the Palazzo Ducale, the Portrait of the Four Tetrarchs indicate superiority as if it were defending its church as well as the Palace. Since it is right next to the Palazzo Ducale, it also nears the mouth of the Grand Canal as well as the place where visitors of the Doge would enter. This shows the power of Venice’s strength, conquers and how they have overcome their enemies. The way they grasp their swords and holding one another expresses solidity, like nothing can break through them. A symbol of duchy’s loyalty to the Byzantine Empire and subsequently of the inheritance is seen through the Tetrarchs as its mantle and continuation of its history.
 Ettore Vio, The Basilica of St. Mark in Venice, (Florence: Scala Group, 1999), p. 62
 Ibid, p. 62