In all of the decisors and commentators and books of legends that bring this saying, it is brought using the language, "It corresponds to the four languages of redemption." But if we look in the Palestinian Talmud from which the phrase is quoted, the word "languages" is missing. Rather it says, "four redemptions." In my view, this is a very significant distinction. For if we understand it according to its plain meaning, the notion of four languages of redemption is not a reason to establish four cups of thanksgiving, since there is but one redemption, whether we speak of it using one word or four. But if there are four redemptions, then that explains the matter much better, for when we examine it well we find that there are four different ideas expressed in these four terms, and each one is its own complete idea unto itself and deserving of its own thanksgiving. In the first statement ("I will take you out"), we see that the Holy Blessed One took them out from under the labors of the Egyptians, which is to say that God lightened their labors--but they were not freed completely. Then God added to this, "And I will save you from their servitude," which means that they did not serve them at all. But even after these two promises, they were still servants to Pharaoh and had not achieved full redemption. Thus God added, "And I will redeem you," but even so they were not yet made a special acquisition of the Holy Blessed One, and so God added "And I will take you to me to be a people, and I will be a god to you." We see that within these sayings are included separate notions of redemption. The verses continue in a "not only this, but also this" fashion--that is to say, not only will I do this, but I will also do this, and not only that, but also this, etc. As such, the simple meaning is revealed: We are obligated to give thanks for each individual act that increased the redemption. And that is why [the Sages] established the four cups of thanksgiving.
And why did they not establish a cup for the phrase "and I will bring you to the land?" We cannot say that it is because that language is not related to the redemption, since they established a cup for "and I will take you to me to be a people," which is likewise not specifically an aspect of redemption. Rather, the simple answer is that since we are still in exile, and the land is in the hands of non-Jews, it is impossible to raise a glass of wine for this. And perhaps, in recognition of this, they established a special cup in the name of Elijah, as a remembrance and hint that we hope for his coming and for the resurrection of the nation and the land speedily in our days.