So I've heard at various points that the east-west divide was unintentionally hurtful towards the west that really started to show in the last decades of the western empire. One solution I've heard is that a four way could have been better for the empire overall.
So my question is what would be the best way to divide the empire in four and when would it be the best time to do this that would provide overall better longevity to the roman world as a whole.
I admit a timeline with this idea interest me.
The empire was divided in four by Diocletian...
Instead of a quadripartite division, I was sketching a TL idea (the PoD being a much earlier death of Constantine and a more successful Gallerius) which proposes a tripartite division: an Western Roman Empire (Hispania + Gallia + Britannia), focused on the Rhine border | the "Central" Roman Empire (Italia + all Africa west of Lybia + the whole of the Balkans), which goes neatly along the Danube border | the "Oriental" Roman Empire (all the Asian provinces + Egypt and Lybia), which should focus on the border with Persia.
Why I think it's better? Simply because it focus on the defensive frontiers is more suitable to permit a longer lived Empire. Despite the worst enemy of Rome arguably being Persia, its IOTL ruin came from the Danube first (the Gothic invasion post-Adrianople) and then from the Rhine, and only because Stilicho stripped the
limes germanicus from its garrisons to face Alaric. And all of this because he was disputing with the Eastern Roman Empire the control of Illyria.
Thus, if the whole of Italy and the Balkans are concerned with the protection of the Danube, the *Gallic/Western Empire (despite being the weakest link of the Roman chain) is left protecting a much smaller border and defensible in the Rhine, and also the border with the Picts - which, until 406 A.D., were considered the safest borders of the Empire. Regarding the East... well they can keep the Sassanids at bay (if even the Palmyrene Empire with a much smaller powerbase managed to contain the Persians). Finally, the Central Empire won't be too concerned with Italy or Africa, but will strive to preserve the
status quo on Pannonia, Moesia and Thracia, just like the Byzantine Empire did until the 7th Century.
Now, if you want a quadripartite division, it will probably be on the lines of the Tetrarchy, perhaps the only difference being that Hispania goes to the Gallic administration instead of the Italian administration.