Nice coin!

Also, great history! Below is a litle more history from: Constantine,
Unconquerered Emperor, Christian Victor, By Paul Stephenson
The First Tetrarchy
"Diocletian, commander of the imperial bodyguard, was acclaimed emperor by his troops near Nicomedia...(Izmit)...in November AD 284..."
"His first act was to dispose Aper, the Praetorian Prefect with whom he had conspired to kill Numerianus , elder son of Carus (282-3). Before May 285 Carus' second son, Carinus, was murdered by his own troops in Moesia to avoid battle with Diocletian..."
"...on 21 July 285, Diocletian took...Maximianus, or Maximian, as his Caesar."
"...Diocletian had no son, only a daughter, whom he later married to Galerius. He needed a general whom he could trust to operate in the west while he campaigned to the east..."
"A deal was struck that Constantius was to be appointed Caesar, Junior emperor, and would remain subordinate to Maximian... Diocletian would also appoint a Caesar, and his choice was his own Praetorian Prefect, Galerius.
"Constantius, although the senior of the two Caesars, was formally adopted into the junior Herculian (Hercules) line." Under Maximian.
"Galerius was adopted into the senior Jovian (Jupiter) line and married Diocletians daughter, but he was the junior Caesar and hence the most junior of the four emperors."
"The situation was complicated further when the Caesars took patron gods...Constantius favouring Mar's,...Galerius...Sol Invictus"
Maximian also had a son (Maxentinius) who was effectively, sidelined. "...some time later Maxentinius was married to the daughter of Galererius. Constantine similarly became a pawn in the great game, dispatched to the east with Diocletian and Galerius, a potent incentive to Constantius to remain loyal..."
"The harmony between the four emperors is illustrated by a porphyry statue carved from purple marble quarried in Egypt... The porphyry statues reflect an ideal of concord and co-operation that certainly did not exist between the Tetrarchs..." These Tetrarchs, "were rarely together...traveled constantly, spending summers in military compounds and winters in various provincial capitals." The emperors were in fact autonomous for the most part. "The regions were, roughly in modern terms:
Dioceltian (Senior AVG, Jovian) in eastern Turkey, Palestine and Syria, and Egypt
Galerius (Junior CAES, Jovian) in western Turkey, the Balkans, and Austria-Hungary
Maximian (Junior AVG, Herculian) in Italy, Spain, and North Africa (Tunisia and Algeria)
Constantius (Senior CAES, Herculian) in northern France and the Benelux countries, southern France and Britain