There are four cups of wine at the Seder: The classic reason we have four is because of four Leshonot Shel Geulah:
הוצאתי היצלתי גאלתי ולקחתי. Another source (Seder Ha'Oruch) is that the four cups represent the four people that have to make a special Bracha to give thanks to Hashem for being saved.  This Bracha is called הגומל, and people say it when they have:

1) Crossed a desert

2) Went over the sea

3) Left prison

4) Were sick and healed.  

When Bnei Yisroel left Mitzrayim, they actually did: cross a desert, go over the sea, leave the prison of Mitzryaim and were physically and spiritually healed.  Therefore we must show Hashem הכרת הטוב -we are giving Him thanks for taking us out of  מצרים. In fact, the entire seder is to show Hakarat Hatov to Hashem. For example, Dayeinu is makir tov every step of the way. Showing gratitude is a very important midda (character trait). Tal Ben Shachar a leader in positive psychology frequently speaks about the importance of expressing gratitude. Psychological research has repeatedly shown that when we are thankful for the good in our life, the good grows and we have more of it. The opposite, sadly, is also true: when we fail to appreciate the good, when we take it for granted, the good depreciates.  It appears that being grateful triggers a positive spiral of growth and well-being. When you think of all you can be grateful for, when you take stock, you feel better. When you feel better, you become more open to—and are more likely to notice and pursue—positive experiences. You can begin this positive spiral of happiness at any moment by choosing to reflect on the things for which you are grateful. So on this special night, at the seder,  take time to think of all those in your lives to whom you should be grateful:  your caring family, your dedicated Rebbeim, Morot and teachers and your amazing friends.  I, personally,  would like to take the opportunity to express my Hakarat Hatov to everyone at Yeshiva Har Torah, the wonderful students, parents, faculty and staff.  Wishing everyone a Chag Kasher V'Sameach.  (Divrei Torah from Rabbi Aaron Leibtag, Morah D'Asra KJBS, Chicago).


haggadah Section: -- Cup #2 & Dayenu