God uses four expressions of redemption in describing our Exodus from Egypt and our birth as a nation:
1. “I will take you out…”
2. “I will save you…”
3. “I will redeem you…”
4. “I will take you as a nation…”
Our Sages instituted that we should drink a cup of wine, a toast if you will, for each one of these expressions. We recite the Kiddush over the first cup, we read the Exodus story from the Haggada over the second cup, we recite the Grace After Meals over the third cup, and we sing the Hallel - Psalms and praises to God - over the fourth cup.
There is actually a fifth expression in the above mentioned verses, "And I will bring you to the land which I promised to Abraham, Isaac, and Jacob, and I will give it to you as an inheritance".
In honor of this verse we have a fifth cup at the Seder: the Cup of Elijah. This cup is set up for Elijah during the second half of the Seder, but we do not drink it.
There are a number of explanations of the significance of the various stages of redemption conveyed through each of these expressions. Here is one:
1. Salvation from harsh labor. This began as soon as the plagues were introduced.
2. Salvation from servitude, on the day the Jews left Egypt geographically and arrived at Ramses.
3. The splitting of the sea, after which the Jews felt completely redeemed, without fear of the Egyptians recapturing them.
4. Becoming a nation at Sinai.
While the Exodus from Egypt and the birth of the Jewish nation were permanent, we have not yet reached the ideal state of perfection. That is why the fifth expression/cup is different than the other four, and it is reserved for Elijah. Elijah will announce the arrival of Mashiach, who will bring all Jews to Israel.