What is the source for the Four Cups?
Rabbi Yoḥanan in the name of Rabbi Benaiah: Corresponding to the four redemptions: … "I shall take you, I shall save you, I shall free you, I shall take you." [Exodus 6:6-7] (Jerusalem Talmud, Pesaḥim 10:1)
The Jerusalem Talmud understands the verse not as presenting four expressions of redemption, rather four actual redemptions, which are stages leading to the ultimate redemption.
Following are my father's comments on the four redemptions.
The first redemption is "I will take you out from under the burdens of the Egyptians," first and foremost, redemption from enslavement necessitates realizing that bondage is a burden and inherently oppressive. God informed His nation that they must be aware of the burden of their slavery and may not be tolerant of enslavement. Typically, a slave dreams that his master is not overly oppressive, but the Divine message is that all slavery is inherently oppressive, even if the master is relatively lenient with his slave.
The basis for achieving freedom is recognizing the intrinsic suffering of slavery, and only with this insight can one advance to the next stage of redemption, "I will save you from their labor." When the Israelites understand that suffering and oppression are inherent in slavery, then God promises to save them from servitude to the Egyptians, to free them from their hard labor, that they be free in body. Only one who is aware that slavery constitutes oppression can fully appreciate his redemption from slavery.
However, physical freedom is insufficient, we must achieve spiritual freedom, recognizing the essential value of freedom. This is the stage of "I will redeem you with an outstretched arm," complete redemption. Through the miracles God will perform in Egypt for His nation, because of God's "outstretched arm" over Egypt, Israel will be redeemed and experience spiritual freedom, which will lead to the fourth stage of redemption, "I will take you to Me as a people," Israel will become God's nation, a holy nation. Further, "I will take you to Me as a people," Israel's acceptance of the yolk of heaven will be the full realization of their freedom, as our Sages teach "none is free, save he who occupies himself with Torah."
"I will bring you to the Land" is not included as one of the redemptions, for the others are permanent, while Israel was destined to be exiled from its Land, though the Promised Land remained the nation's homeland, even when they did not dwell within her. However, in its future redemption, the Nation of Israel will be brought back to its Land permanently, at which time "I will bring you to the Land" will be fixed as the fifth redemption. (David Magence)

Shabbat Shalom!
The Va’ad