Siman 480

And after they have drunk the third cup, it is the custom to say "Pour out Your wrath...", and to open the door in order to remember that it is a night of guarding. And in the merit of this faith the Mashiach will come, and He will pour out His wrath upon the Babylonians who destroyed the Beit HaMikdash. And because before the coming of the Mashiach, Eliyahu will come, as it is written: "Behold, I send to you Eliyahu the prophet before...". Therefore, the custom is to place another cup in the middle of the table, and we call it "the cup of Eliyahu." And there are those who were accustomed to stand from the reclining position and say "Welcome." And it is all to strengthen faith, for so we have received by tradition that in Nisan they were redeemed – and in Nisan they are destined to be redeemed, as it is brought in the first chapter of Rosh Hashanah (11a). Therefore, at the time of the conclusion of the Seder before saying "Not unto us... but unto Your name give glory" – we awaken ourselves so that He shall give glory to His great name, may He be blessed.

And they pour the fourth cup, and he begins "Not unto us," and finishes the Hallel. And when he reaches "They shall praise You," he says the Great Hallel, which is the psalm of twenty-six "Give thanks" until its end. And afterward he says "Nishmat," and it is called "the blessing of the song" in the Gemara (118a) because on Shabbatot and Yom Tov they say it after Pesukei D'Zimrah (Tosafot ibid. starting with "Mai"). And these twenty-six "Give thanks" correspond to the twenty-six generations before the giving of the Torah, whom the Holy One, blessed be He, sustained with His kindness (Gemara ibid.). And it is called "the Great Hallel" because it is written therein "Who gives bread to all flesh" (ibid.), for the Holy One, blessed be He, sits at the height of the universe and distributes sustenance to every creature (ibid.). And because on Pesach we entered under the wings of the Shekhinah, and He promised us that He would not leave us nor forsake us forever—therefore we say this in the order.

Regarding the conclusion of the Hallel, opinions are numerous: our teacher, the Beit Yosef, wrote that one says "Nishmat" and "Yishtabach" until "for from eternity to eternity You are G-d." Afterward, he says "Yehalelukha" and concludes with "King extolled with praises." The Tur wrote that "Yehalelukha" should be said after Hallel until the blessing, and afterward "Nishmat" and "Yishtabach," and he concludes with "Yishtabach"; the Tosafot wrote similarly ibid. We follow the practice of our teacher, the Beit Yosef. After the blessing, one recites "borei peri ha-gafen" over the fourth cup, because he interrupted with Hallel. Our teacher, the Beit Yosef, ruled without a blessing, but we do not practice accordingly, as I wrote in siman 474. He drinks while in hasabah, and if he did not perform hasabah, it has been explained in siman 472. He should drink all of it in order to recite the final blessing "al ha-gafen...". Afterward, he says the piyyutim according to the custom. There are those who do not drink until after the recitation of the piyyutim, and it is printed thus in the Haggadot. This is not correct, for the fourth cup must be drunk immediately after Hallel. (The Beit Yosef wrote: If he drank without hasabah—he must return and drink with hasabah. Then he must recite "borei peri ha-gafen" before it, because he diverted his mind from further drinking. Thus far his words. Above in siman 472, we brought opinions that he does not need to return and drink with hasabah, see there. The Rema wrote that Hallel may be said even not in the place of the meal, see there. Nevertheless, it is better to say it in the place of the meal, and to drink all the cups in one place.)