The Sages instituted drinking four cups of wine on the Seder night in order to increase the joy of redemption and give expression to our freedom. On every Yom Tov there is a mitzva to rejoice by drinking wine, but for Pesaĥ the Sages further integrated four cups of wine into the Seder, so that our joy finds expression in each of its phases. Kiddush is recited over the first cup, and thus everyone’s cup is filled prior to kiddush. The story of the Exodus and the first part of Hallel are recited over the second cup, which is therefore filled just before the telling of the story is begun. Birkat Ha-mazon is recited over the third cup; we refill our glasses prior to its recitation and drink the wine right after. Finally, we pour the fourth cup, recite the second part of Hallel and “the Great Hallel” (see below section 35) over it, and then drink the cup. Thus, every recitation at the Seder is over wine.
If one drinks four cups of wine one after another, it is as if he drank only one cup (SA 472:8). Even if one waited between cups, if he did not recite any of the Hagada during these pauses, he has not fulfilled this obligation according to several poskim (Rashbam, Ran, Pri Ĥadash). This is because one must drink while discussing the Exodus. According to Beit Yosef, however, if one pauses between cups he fulfills his obligation be-di’avad (BHL ad loc. s.v. “she-lo”).
The Sages explain the four cups as alluding to several things: the four expressions of redemption used in the Torah’s account of the Exodus; the four kingdoms that subjugated Israel after it became a nation (Babylon, Persia, Greece, and Rome), and from which the Almighty saved us; the four cups of calamity that God will serve to the wicked among the nations of the world; and the corresponding four cups of consolation that God will pour out to Israel (y. Pesaĥim 10:1).
As a rule, the number four represents completeness, for everything in the world has four sides, corresponding to the four points of the compass. Since the Exodus brought about a complete upheaval in the world, the Torah uses four expressions of redemption in relation to it:
Therefore, say to the Israelites: “I am the Lord. I will rescue you from beneath the burden of Egypt; I will save you from their enslavement; I will redeem you with an outstretched arm and great acts of judgment; and I will take you for Myself as a people and be your God. Thus you will know that I am the Lord your God, Who is taking you out from beneath the burden of Egypt.” (Shemot 6:6-7)
Israel’s bondage in Egypt was more than just the enslavement of those 600,000 Jews. It manifested the subjugation of the spirit to the material, because the possibility of expressing spirituality in the world depends on the people of Israel, who were enslaved by the most materialistic of kingdoms, Egypt. In order to free Israel so that they could receive the Torah and illuminate and rectify the world, it was necessary to break all barriers of oppression, from each direction. The four expressions of redemption correspond to these.
In fact, a fifth expression of redemption appears in the very next verse: “I will bring you to the land that I swore to give to Avraham, Yitzĥak, and Yaakov. I will give it to you as an inheritance; I am the Lord” (ibid. 8). Since this verse does not address the Exodus itself, the Sages did not institute a corresponding fifth cup. Nonetheless, it is customary to pour a fifth cup, known as Eliyahu’s Cup (“Kos shel Eliyahu”), which alludes to the complete redemption that begins with entry into the Promised Land (see section 35 below).