Parashas Vaeira
לָכֵ֞ן אֱמֹ֥ר לִבְנֵֽי־יִשְׂרָאֵל֮ אֲנִ֣י יְהֹוָה֒ וְהוֹצֵאתִ֣י אֶתְכֶ֗ם מִתַּ֙חַת֙ סִבְלֹ֣ת מִצְרַ֔יִם וְהִצַּלְתִּ֥י אֶתְכֶ֖ם מֵעֲבֹדָתָ֑ם וְגָאַלְתִּ֤י אֶתְכֶם֙ בִּזְר֣וֹעַ נְטוּיָ֔ה וּבִשְׁפָטִ֖ים גְּדֹלִֽים׃
Say, therefore, to the Israelite people: I am GOD. I will free you from the labors of the Egyptians and deliver you from their bondage. I will redeem you with an outstretched arm and through extraordinary chastisements.
Torah Temimah points to R’ Yochanan — Yerushalmi Pesachim / TT (verified).
והוצאתי אתכם וגו׳ — מניין לארבע כוסות בפסח, א״ר יוחנן: כנגד ארבע גאולות, והוצאתי והצלתי וגאלתי ולקחתי.
English: “And I will bring you out\ldots” — From where [do we know] the four cups at Pesach? R’ Yochanan said: Corresponding to four redemptions — “I will bring out,” “I will deliver,” “I will redeem,” and “I will take.”
Custom
Chazal derive the four cups of the Seder from the four expressions of redemption in these verses.