According to the Talmud Yerushalmi (Pesachim 10:1), the four cups of wine that we drink at the Pesach seder, correspond to the four expressions of redemption including, "I will take you out", "I will deliver you", "I will redeem you", and "I will acquire you" (Shemos 6:6-7). This is to extenuate that our redemption from Mitzrayim took place in phases and entailed four distinct aspects, namely, our geographical removal from the land of Mitzrayim, our physical freedom from the bonds of slavery, our emergence as an emotionally liberated nation, and our spiritual designation as the chosen people. However, some rishonim cite an alternative version of the Gemara (Pesachim 118a) which records the existence of a fifth cup of wine. The Raavad in his commentary to the Rif (Pesachim 26b) as well as the Daas Zekeinim m'Baalei Ha'Tosfos (Shemos 12:8) claim that this additional cup reflects a fifth manifestation of the redemption, "and I will bring you into the land" (Shemos 6:8), referring to our development as a sovereign state in the Land of Israel.
Since the rishonim debate the legitimacy of this text of the Gemara, and the existence of a fifth cup altogether, as well as the permissibility of drinking extraneous amounts of wine at the end of the Pesach seder, the fifth cup is traditionally only poured but not imbibed. The Vilna Gaon asserts that this compromise is the basis for our practice regarding the cup of Eliyahu, which is ceremoniously filled towards the conclusion of the seder, but is left untouched, for it is Eliyahu who will resolve all Talmudic disputes including the validity or necessity of a fifth cup of wine. However, perhaps the fifth cup is ascribed to Eliyahu, not only because he alone will resolve its halachically controversial status, but also because it is associated with the culmination of our ultimate redemption and the resettlement of the Land of Israel in the times of Moshiach which will be heralded and preceded by the return of Eliyahu hanavi.
Indeed, Rabbi Meir Horowitz of Dzikov (Imrei Noam) notes that the numerical value of the fifth term of deliverance, "and I will bring you" - "ve'heveisi" - is four hundred and twenty four, and equal to that of "Moshiach ben David", which further underscores that this expression is connected with the coming of Moshiach and the future redemption. For this reason, the fifth cup is prepared but not consumed, because despite our confidence in the final geulah, we can only rejoice and toast its arrival after it has materialized. This is supported by Rav Chaim Soloveitchik's (quoted in Emek Bracha) interpretation of the pasuk, "But I have trusted in Your mercy, my heart shall rejoice in Your salvation, I will sing to the Lord for He has dealt bountifully with me" (Tehillim 13:6), where he explains that while we can preemptively "rejoice in God's salvation" since it is inevitable, it is only appropriate to celebrate externally and sing to the Lord, or raise our glasses in triumph, after the geulah has actually occurred.
Nevertheless, even though the fifth cup of wine cannot presently be enjoyed, it must be prepared, in order to demonstrate our persistent yearning for the advent of Moshiach. The fifth cup serves as a symbol of our eternal hope and constant anticipation of the final geulah, which enables us to withstand what can appear to be an interminable galus. Yaakov Avinu summoned his children at the end of his life and attempted to disclose to them the precise date of the end of days. His plan was thwarted by Hashem, but his intentions were clear, as the ensuing pasuk states, "For your salvation I hope Hashem" (Breishis 49:18). Yaakov understood that in order to persevere throughout the long journey of galus Mitzrayim, Klal Yisrael would need to be able to hope and foresee a brighter future for themselves. In fact, the Kozhnitzer Maggid (Avodas Yisrael) notes that from the start of Parshas Mikeitz until this pasuk in Parshas Vayechi the name of Havaya - which reflects the merciful aspect of Hashem's providence - is entirely absent. This is because Parshas Mikeitz marked the beginning of a prolonged period of galus, which ostensibly eclipsed any trace of mercy or compassion. However, through the penetrating power of hope it is possible to pierce through even heaviest veil of darkness, and to begin to discern the infinite mercy of Hashem.
It seems that galus is only manageable when there is at least a glimmer of hope for redemption. For this reason, none of the cities of refuge, where an accidental murderer could be exiled and protected, were operational prior to the final conquest of Eretz Yisrael. The Meshech Chochmah (Masei) explains that this is because the detention of the accidental murder can only be alleviated and dismissed by the death of the Kohen Gadol, and Hashem had already pledged that Ahron's successor, Eliezer Hakohen, would live to oversee the entire process of conquering and dividing Eretz Yisrael. Therefore, during this transitional stage of Jewish history, any accidental murderer sentenced to galus would have had no potential path towards parole or liberation. This kind of bleak existence is so harsh and excruciating that it could not be justified as a punishment for any crime committed inadvertently.
For this reason, at the very same moment that Hashem informed Avraham that, "your descendants will be strangers in a land that is not theirs, and they will be enslaved and oppressed for four hundred years" (Breishis 14:13), Hashem also foretold that, "the nation that they will serve, I will judge, and afterwards they will go forth with great possessions" (Breishis 14:14). Why was it necessary to foreshadow to the manner of their emancipation before the period of slavery had even begun? Rav Yerucham Levovitz (Daas Torah) suggests that this relationship underscores that our ability to endure the difficulties of galus is directly dependent on the prospect and promise of salvation. Therefore, the Rambam in his commentary on the Mishnah (Sanhedrin 10:1) demands that we not only believe in the coming of Moshiach, but that we also resolve to "wait for him every day." It is this kind of optimistic mindset, our hope for deliverance and rescue, that will propel us to the time of our true redemption when we can rejoice fully and enjoy all five cups of wine at the seder, with the rebuilding of the Beis Hamikdash be'meheirah be'yameinu!