The Foundation Stone Haggadah: Themes of the Haggadah 8
The Vilna Gaon says that the number that occurs over and over on Pesach night is the number four. We have four cups of wine, four sons, four questions, etc. Why is four so repeated? He answers that there are four types of people who are obligated to say Hagomel. They are: someone who was sick and was healed, one who was in prison and was freed, one who traveled through the desert and survived, and one who traveled over water. In participating in the Seder, each one of us is one of those people who must bring a Thanksgiving offering. Since we see ourselves as if the miracle is occurring now, we use the number four to represent Hagomel to us, instead of saying Al Hanisim.