There are several reasons why we drink four cups of wine on Passover. One is to symbolize Pharoh's four decrees and another is to symbolize God's four promises. In my opinion, God's four promises would be best to tell why we drink four cups of wine. God's four promises are "I shall take you out. I shall rescue you. I shall redeem you. I shall take you to Me." The reason why I would prefer this is because the whole point of the story is not to tell what Pharoh did to the slaves and how he punished them. It is to tell us that God saved us from slaver and brought us to the promised land. So, in my mind, the four cups of wine represent how God rescued us and made us His people.