Symbolism of the Four Cups

The First Cup

1. Said over Kiddush
2. Corresponds to this world, to signal that we should sanctify ourselves in this world.
3. Corresponds to the word "והוצאתי", "and I will take you out"
4. Corresponds to the Geulah of 56, the Israeli War of Independence

The Second Cup

1. Said over Magid
2. Corresponds to Yemot HaMoshiach, when we will still mention Yetziat Mitzraim
3. Corresponds to the word "והצלתי", "and I will save you"
4. Corresponds to the Geulah of 60, the Six-Day War, the Pekidah.

The Third Cup

1. Said over Birkat HaMazon
2. Corresponds to Atid Lavo, when we will have Seudat Livyatan
3. Corresponds to the word "וגאלתי" "and I will redeem", but on the side of Chesed, "בזרוע נטויה", "with an outstretched arm"
4. Corresponds to the Geulah of 66, with Moshiach ben Yosef, followed by the Zechirah.

The Fourth Cup

1. Said over the Great Hallel
2. Corresponds to Techiyat HaMeitim, when we will say the Great Hallel
3. Corresponds to the word "וגאלתי" "and I will redeem", but on the side of Din, "ובשפטים גדולים" "with great judgments
4. Corresponds to the Geulah of 72, with Moshiach ben David

Notes

Numbers 2 and 3 of every list are according to the GR"A, who held that the common relationship between expressions of Geulah and the cups was incorrect, but rather it was all from a single pasuk:

שמות ו:ו - לכן אמר לבני ישראל, אני ה', והוצאתי אתכם מתחת סבלת מצרים, והצלתי אתכם מעבדתם, וגאלתי אתכם בזרוע נטויה, ובשפטים גדלים

Therefore, the question of whether or not five cups are needed goes away.

Number 4 is according to the Zohar Raya Mehemna on Pinchas.  On the first and second cups, the correlation between 3 and 4 works well.  "And I will take you out" is well suited for the Kibbutz Galuyot of 600,000 achieved at the time of the war in 5708.  The threat of immenent destruction that preceded the war in 5727 goes well with "And I will save you".  The question is how Moshiach ben Yosef has to do with the Geulah on the side of Chesed, and Moshiach ben David with Geulah on the side of Din.  Perhaps the answer is that Moshiach ben Yosef will come in the zechut of the work of Am Yisrael in the Keitz Meguleh, and therefore the experience will be sweeter, whereas the Geulah of Moshiach ben David will be one of more Din, as it is related to the final war of Gog uMagog.

For more symbolism, see Yerushalmi Pesachim 10:1.