The Four Cups of the Seder are structurally connected to the four verbal performances this evening:
(1) Kiddush, sanctifying the holiday
(2) Maggid, the storytelling
(3) Birkat HaMazon, completing the Pesach meal; and
(4) Hallel, completing the festival Psalms.
The Talmud connects the Four Cups to God's Four Promises to Israel: "Tell the children of Israel: I am Adonai! I will take them out... I will rescue them… I will redeem them… and I will marry them taking them as my...