(Mishnah): They pour him the third cup and he recites upon it Birchas HaMazon.

Upon the fourth cup he completes the recital of Hallel and recites Birchas HaShir (which refers to either 'Nishmas Kol Chai' or 'Yehalelucha Hash-m Kol Ma'asecha').

One may drink between these (earlier) cups but one may not drink between the third and fourth cups.

One may not conclude the meal with Afikoman (the Gemara will explain this term).

(Gemara): The Pasuk states (in the Song of Devorah the prophetess (Shoftim 5, 2), "When revenge is brought upon Yisrael and the nation devotes itself (to Hash-m), bless Hash-m.'' When Hash-m performs a miracle for Yisrael (by inflicting vengeance on the enemies of Yisrael), you should sing songs of praise to Him.

Question: When Yisrael left Egypt, they did not sing a song of praise (until they crossed the Red Sea)?

Answer: The actual Exodus was only the beginning of their redemption (which culminated in crossing the Red Sea).

Question: When the Miracle of Purim occurred to Mordechai and Ester, they did not sing a song of praise?

Answer #1: As they were in Chutz La'aretz (and after the time when Yisrael came to the land, we no longer recite a song of praise for a miracle that occurred outside of Eretz Yisrael).

Answer #2: Mordechai and Ester were redeemed from their enemies but not from the nation that governed them.