?What is the Saying? Amaturers study Tactic, Professionals study Logistics.Would it be possible to have a surviving wanked Western Roman Empire, a surviving wanked Eastern Roman Empire, a huge Chinese dynasty that has essentially become the analogue to the Mongol Empire by conquering most of Asia , and a super Gupta Empire that has almost all of India and some of Persia, all in one war?
Fredrick the Great's forces could fit in one side of a college football stadium, and most of his opponents would fit on the other side.
My Guessamate is that no historical political unit pre 1600's could support more than 200~250,000 Soldiers on Campaign [Garrison Soldiers are different].
So given your 4 Empires -- my guess is max of 3/4 ~ 1 Million Troops in any war.