Question

At what historical point can it be identified that the four cups of wine were a part of the Passover meal?

Answer

The four cups are from Mishna Pesachim 10:1. It is a rabbinical decree , based on the four expressions of redemption in Exodus 6:6-7 - (source: Talmud Yerushalmi 10:1).

Apparently, it was instituted in the time of the Great Assembly ("Anshei Knesset HaGedolah"), whose period extended from Ezra to the beginning of the Tannaic era.