4.5 million at turn of century means minimum of 13-15 million modern day in the current territories. Add Cyprus , thessolonika, Constantinople and other otl lands and you suddenly have a state which in modern times has perhaps 25-30 million people. A clear regional power , especially if it continues to have good governance.
Think otl Australia for a best case for Greece
I think best case scenario for Greece territory wise would be taking a very limited Anatolian Coast and then doing a population exchange(ie Cappadocian and Pontic Greeks are deported to the Anatolian Coast and Turks in these territories are deported to Turkey)