Hows Vietnam doing? I want to see more of the Japanese puppets in Asia.You are right- he wouldn't technically have a number. However, since Otto is five years old at this point, Maximilian will be running the empire for thirteen years (and he'll likely be a power behind the throne for far longer). Maximilian was winning the war while Otto was playing with lead soldiers, as it were. So Maximilian is, for all intents and purposes, the emperor- the fourth Habsburg emperor named Maximilian. Thus, calling him "Maximilian IV", even though it's technically cheating, makes sense (and it's likely how he'd be referred to in practice by the common people).
TL;DR: Since Maximilian is really running the place, he de facto gets the number.