Commentary for Pesachim 235:16

We read it thus: 'on the fourth, one completes hallel and recites the great hallel.'

Our rabbis learned, "We say the Great Hallel (Psalm 136, and see below at the bottom of the paragraph for other opinions) over a fifth cup - these are the words of Rabbi Tarfon, and there are those that say, 'The Lord is my shepherd, I shall not lack' (Psalms 23)." And Rashbam [follows] the textual variant, "We say the Great Hallel on the fourth cup," as 'who mentioned the name' of the fifth cup; and if it is coming to say [that the fifth cup is optional and, if wants, he should do a fifth cup, this is how it should have said it: "One who wants to do a fifth cup should say the Great Hallel upon it." But according to the textual variant of the books, it is implied that for Rabbi Tarfon and the 'those that say,' the fifth cup is obligatory, but the world has become accustomed to make it optional. And so did Rabbi Yosef Tov Eelem, may his memory be blessed, write - that if desires to drink, he should make a fifth cup. It is implied from his words that it is [otherwise] forbidden to drink after the four cups. And it is a wonder, as from where do we have this; as later it only says, "We may not eat an afikoman (a dessert or other foods eaten after the meal) after [we are finished eating] the Pesach sacrifice" - which means not to eat, but to drink is permitted. And so also since in the [Talmud] Yerushalmi, it explains that he should not drink between the third and fourth [cups] because of 'lest he get drunk,' it is implied that if we were not concerned about drunkenness, it would be permissible to drink. But the custom that has spread is not to drink wine. And Ram, may his memory be blessed, explained a reason for this custom: that it is because a person is obligated to be involved the whole night with the laws of Pesach and with the exodus from Egypt, to recount the miracles and wonders that the Holy One, blessed be He, did for us, until slumber overcomes him - and if he drinks, he will get drunk [and not be able to do this]. And so too do we say in Tosefta Pesachim 10:11, "A person is obligated to be involved with the laws of Pesach and with the exodus from Egypt the whole night"; and this is what we learned (Pesach Haggadah, Magid, Story of the Five Rabbis 1), "It happened once [on Pesach] that Rabbi Eliezer, Rabbi Yehoshua [...] were reclining in Bnei Brak and were telling the story of the exodus from Egypt that whole night, etc." But with the fourth cup, they permitted [drinking], as it has scriptural support, corresponding to the verse (Exodus 6:7), "and I will take you." From where is the Great Hallel? Rav Yehuda said from "Praised" (Psalms 136:1) to "On the rivers of Babylon" (Psalms 137:1). Rabbi Yochanan said from "A song of ascents" to "On the rivers of Babylon." Rab Achaa bar Yaakov says from "Since the Lord has chosen Yaakov" (Psalms 135:4) to "On the rivers of Babylon." But the custom of the world is like Rav Yehuda concerning the Great Hallel.