Four cups of wine are to be consumed during the Seder. They are not to be drunk at once, but one at a time at specific points of the Seder and in a proscribed order. The first cup opens the Seder and is called the Kiddush. The second cup is drunk immediately preceding the second Washing of Hands. The third cup is drunk at the end of Grace After Meals and immediately preceding the invitation to Elijah to enter the home. The fourth and final cup is drunk at the end of the Hallel. Each time the celebrants drink a cup of wine they precede it with a blessing, pronounced while raising the cup. This custom is missing from older editions of the Haggadah and books that deal with Ashkenazi rituals. It apparently originated with Rabbi Isaac Luria (16th Century c.e.), and has since been accepted into many rituals.
The Mishnah gives several specific directions governing the consumption of the four cups of wine. First, it is incumbent on each Jew to drink the four cups of wine during the Seder regardless of their economic standing. "A man is duty bound to sell his clothes, or to borrow money, or to hire himself out to obtain wine for the four cups." Second, red wine is preferred because of its superior quality. However, because of the blood libel accusations during the Middle Ages, it has become acceptable, but still not preferred, to use white wine. Third, each cup must contain ¼ log (0.137 liter).
The generally accepted explanation for why there are four cups of wine is that each cup represents one of the four expressions of redemption or deliverance to the Israelites. "I will bring you out . . . I will deliver you . . . I will redeem you . . . I will take you to me for a people." In addition, Mordell Klein points out that there is a precedence of using sets of four in Jewish traditions. For example, there are four tzitzit, or fringes, worn on the tallit, four passages of the Torah in the phylacteries, and four kinds of plants are taken and waved on Sukkot (Festival of Tabernacles). The number four plays a prominent role in other aspects of the Seder as well. The youngest child asks four questions to instigate the recantation of the Passover Story. There are four types of sons described, and there are four pieces of matzah (initially there are only three, but once the Afikomen is divided into two pieces, the three become four).
While it is clear that four cups of wine are to be drunk during the Seder, there is controversy over whether a fifth should be added. There are still some Yemenite communities that drink five cups of wine aside from the cup of Elijah. This cup is drunk between the third and fourth cups at the end of the Hallel. The debate stems from a fifth expression of redemption pointed out by Rabbi Tarfon. "I will bring you in unto the land..." However, this expression is unique from the others as it does not express deliverance, but rather promises a land for inheritance after deliverance has already been granted.
The common solution to this controversy has been to set out a special cup of wine for the prophet Elijah. At the appropriate time of the Seder, immediately preceding the Hallel, some families will send their children to the door to invite Elijah in. While they are gone, everyone watches the cup of Elijah very carefully to see if he has come and sipped from his cup. Some families even follow the practice of having one from the company drink the wine in the cup so when the children return, the wine is gone as though Elijah had entered and demonstrated his participation in the Seder.
It is significant to note that the second cup is not completely full because some of it has been spilled in drops while enumerating the ten plagues inflicted upon the Egyptians. This expresses the Israelites' sympathy for the Egyptians who lost their lives in the plagues, particularly the slaying of the firstborn, and for the armies of Pharaoh who perished in the Red Sea when Israel attained freedom. According to Jewish tradition, the hosts of heaven gathered around the Red Sea to watch the crossing of the Israelites. When they saw that God caused the waters of the Red Sea to part for the Israelites and swallow the armies of Pharaoh, the hosts of heaven rejoiced and sang praises to Him. However, He quickly silenced them and asked how they could be so joyful while His children were at that very moment dieing. Such feeling is reminiscent of the layered and overlapping emotions represented when the bitter herbs are combined with the sweet charoseth.
Glatzer, Nahum N. The Passover Haggadah (New York: Schocken Books
Inc., 1953) 10-11
Shulhan Arukh, Orah Hayyim 472:13
Shulhan Arukh, Orah Hayyim
Shulhan Arukh, Orah Hayyim 472:9
Klein, Mordell ed. Passover (Philadelphia: Jewish Publication Society
of America, 1973) 61-64
 Silverman, Rabbi Morris, ed. Passover Haggadah: New Translation with explanatory notes and original readings (Hartford: Prayer Book Press, 1959) 32