“The Cup of Eliyahu” (Minhag Ashkenaz) 

There is a fifth cup of wine which is referred to as “The Cup of Eliyahu.”  There is a dispute in the Talmud as to the necessity of a fifth cup.  Rabbi Tarfon is of the opinion that one should drink a fifth cup in deference to the fifth expression of redemption, “והבאתי,” “And I will bring you.”  Rabbi Tarfon’s opinion was not accepted.  Hazal ruled, however, that since we are in doubt regarding the halachah, we should pour a fifth cup, but not drink from it.  When Eliyahu Ha’Navi comes, he will render decisions that will resolve all our doubts.  He will likewise clarify this question. Thus, this cup is appropriately called, “the Cup of Eliyahu.” (Peninim Haggadah)