The GRA explains that there are four expressions of redemption so we have four cups but we also provide a cup for Eliyahu Hanovi. He says that there is a fifth expression to bring us to Eretz Yisroel which is temporary hoping for permanence, whether this is counted in the expressions is dependent on the ruling of Eliyahu Hanovi. Eliyahu will determine whether it is counted or not, therefore it is called the cup of Eliyahu.
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