The Four Cups of Wine
The Four Questions
The Four Sons
The Four Expressions of Redemption
Why the number "Four" is given this prominence, and what its special connection with Pesach is, is not obvious.
Perhaps it is because of the four seasons, and the fact that Pesach is the Holiday of Spring, the season of rebirth and renewal, the birth of the Jewish People with its liberation from Egypt and its march towards Sinai. There the L-rd would accomplish the purpose of the Exodus, giving the Jewish People and, through them to all of humanity, the Torah, wherein is spelled out G-d's instructions for a way of life which is moral and holy, which will accomplish ultimately the rebirth and renewal of humanity.
Or perhaps it is the connection to the four "Imahot," the "Mothers" of the Jewish People: Sarah, Rivkah, Rachel and Leah, and the wives and mothers in Egypt and throughout Jewish History, who have kept the spirit of the People alive.