Traditionally we have been taught that the four cups of wine which are drunk at strategic points during the seder represent the four different expressions of redemption. There are other suggestions such as the cup of Pharaoh mentioned four times in the Bible or the four cups represent the four different kingdoms which ruled over Israel. The Mishna in Pesachim states that the four cups of wine represent the four different quarters of the haggadah. In reality the four cups were an innovation by Rabbi Yehudah Hanasi. He based it on the ruling that charity given before the Passover holiday among other things ought to consist of the equivalent quantity of four cups of wine. It seems that that quantity would be appropriate for a festive meal. Rabbi Yehudah Hanasi therefore reasoned that if it was sufficient for charity then it ought to be the requisite amount for all people sitting down to the seder.
The rabbis were particular about attaching blessings to each of the four cups. Apparently there was concern that as the Greco-Roman rituals always included wine and thus the tendency towards drunkenness, our rabbis wanted to avoid this phenomenon. By associating the drinking of the wine with a religious rite the chance for bawdiness would be significantly reduced.