I’ve just about finished reading Scott Hahn’s book “A Father Who Keeps His Promises”. I just read about the 4th Cup the other night and I was amazed with his insight into this.
The third cup was called the Cup of Blessing and this is the cup that all Catholics partake in at the Mass. Paul refers to this “cup of blessing” in 1 Corinthians 10. The fourth cup was called the “Cup of Consummation”. During the Passover, the priest would slaughter the lamb - Jesus was both the priest and lamb in His sacrifice. The Douay Rheims Bible translates “It is finished” as “It is consummated.” The passover for Jesus ended at the cross when he drank the sour wine (vinegar) and died.
I would highly recommend this book - its a great introduction to Covenental Theology and provides the proper perspective to approaching Scripture.