A significant uncertainty arose concerning the fifth cup. Some say that there is an extra special mitzva to drink a fifth cup; the fourth cup should be drunk at the end of the Hallel and the fifth cup after the concluding berakha. Others say that the fifth cup is merely the Sages’ recommendation for one who wishes to continue drinking after the fourth cup. Still others say it is forbidden to drink a fifth cup.
The customary practice is not to drink a fifth cup, though the custom is to pour it and to call it “Eliyahu’s Cup.” The Vilna Gaon explains how it got this name: when there is an uncertainty that cannot be resolved, we believe that when the prophet Eliyahu returns as a harbinger of the messianic era, he will resolve it. Thus, we pour a fifth cup in his honor, and when he arrives he will tell us if we must drink it.
We can interpret this issue in a deeper way as well. The Sages instituted the four cups to signify the four expressions of redemption used in reference to the Exodus from Egypt: “I will rescue you…I will save you…I will redeem you…I will take you…” (Shemot 6:6-7). An additional expression of redemption is mentioned there: “I will bring you to the land” (ibid. 8). However, since this does not relate to the Exodus itself, the Sages do not obligate us to drink a corresponding fifth cup. They tell us, however, that there is a mitzva to drink a fifth cup in order to allude to the complete redemption, which begins with our entry into Eretz Yisrael.
It could also be that the uncertainty about the fifth cup stems from the question as to whether it is proper to drink a fifth cup after the destruction of the Temple and during the long exile. Perhaps after the Temple’s destruction we can only celebrate with those cups that allude to our Exodus from the Egyptian bondage, since this will forever distinguish us. Even when the nations of the world subjugate our bodies, our souls remain eternally free; ever since the Exodus, it has been clear that we are God’s uniquely chosen people, that we received the Torah, and that all the hardships that have come upon us have not broken our faith in God, our Redeemer. Therefore, we drink four cups of wine corresponding to the four expressions of redemption from Egypt. The fifth cup, though, does not correspond to our emancipation alone; rather, it alludes to the complete redemption, which depends on our entry into Eretz Yisrael, where the word of God is revealed in all spheres of life, through the Torah and prophecy, and through God’s blessing, which inheres in the building of the nation and the land. This notion of the Temple is that it joins heaven and earth and reveals the divine unity that nourishes everything. Indeed, the number five alludes to the inner, unifying point at the center of the four compass directions. The uncertainty about the fifth cup lies in the question: Is it fitting, in light of the Temple’s destruction, to drink the fifth cup, which alludes to the complete redemption?
The solution is to pour a fifth cup but not drink it as part of the Seder until Eliyahu appears. His very appearance will show us that the time has come to drink the fifth cup, celebrating our complete redemption.
The custom is to pour Eliyahu’s Cup after drinking the third cup; when we pour the fourth cup for everybody, we pour a cup for Eliyahu as well. The custom is to leave Eliyahu’s Cup covered until morning, when we pour the wine back into the bottle and then use it for the morning kiddush.
. Some have the custom of pouring the wine from Eliyahu’s Cup back into the bottle right after the Seder (this is the custom of Chabad). Others have the custom to add the wine from Eliyahu’s Cup to the fourth cup.
I wrote that the fifth cup alludes to a higher level. This is what Maharal wrote in Gevurot Hashem at the end of a brief section on the laws of Pesaḥ. He cryptically states that the fifth cup symbolizes livelihood that comes from God. See R. Goren’s Torat Ha-Shabbat Ve-hamo’ed, pp. 145-154, where he explains that the fifth cup represents the mitzva of settling Eretz Yisrael, which requires total devotion on our part. R. Goren states that the fifth cup is not an obligation because it is on a higher level than ordinary obligations. He encourages drinking the fifth cup nowadays, since we are actively involved in settling Eretz Yisrael.