I've heard there is an opinion that the daled kosos (four cups of wine) at the Passover Seder are m'doraisa (a Torah obligation). What is the source for this, if there is one? I vaguely remember a source being in the Yerushalmi?
What Seforim speak about this? Are there any nafkei minah that are brought or discussed?
Some commentators maintain that, according to the Talmud Yerushalmi, the obligation of the ארבע כוסות (Four Cups) is מן התורה (a Torah obligation). This is the view of the Kli Chemdah (Parshas Va'era) based on the Yerushalmi in Shekalim (3:2), Nazir (6:5), and Pesachim (10:1, contra the Pnei Moshe's interpretation).
To quote one example, the Yerushalmi in Shekalim questions whether one can fulfill the mitzvah of ארבע כוסות with wine of שביעית. Following the interpretation of the Yerushalmi that this refers to fruit after the time of ביעור, the Kli Chemdah explains that the Yerushalmi's question is whether the mitvah of ארבע כוסות can be דוחה the prohibition of eating פירות שביעית אחר זמן ביעור. This possibility could only be entertained if the obligation of ארבע כוסות is דאורייתא.
During the shabbos hagadol drasha this year (5774) our rav mentioned the statement that each of the arba kosos is regarded as a separate mitzva. He stated that many mitzvos must be performed 'al hakos'. Thus the cups themselves are not mi'd'oraisa, but the mitzva they are drunk for are torah prescribed. Other mitzvos that are required al hakos are bris milah, birchat hamazon, or kiddush hayom.
The four mitzvos in hagaddah are kiddush hayom, magid, birchat hamazon, and hallel. Each mizvah has a bracha said al hakos for that mitzvah.
In the case of the question as asked it would appear that the answer would be "no", in one sense, and "yes" in another sense. That is, it is not the matter of the cups themselves that is from the torah, but the associated mitzva. Then again, perhaps the association may be what is being asked about (that is is the association of kos with the mitzva from the torah)
According to the Rambam, Chametz UMatzah 7:11 (in the Mechon Mamre order) there are two distinct obligations in the four cups, the obligation of the four cups (a cup of blessing on each of the four Mitzvos of Kiddush, Haggadah, Birchas HaMazon and Hallel) and the obligation of showing oneself to be free. And the Rambam explains that there are ways to end up drinking the four cups in a way to fulfill one of those without the other.
The four cups aspect is Rabbinic, however the freedom aspect is easily understood as a Torah obligation, derived from specific Pesukim (Rambam there Halacha 8).
So by drinking the four cups mixed to be sweet to the taste of the drinker is a specific way to fulfill the Torah obligation of showing oneself to be free.
[It seems pretty obvious that the Torah requirement could be fulfilled in other ways, but that this is a way to do it, and the Rabbis (according to the Rambam, and the Rif agrees) specifically instituted this way to do it. Much like we say hearing Parshas Zachor is a Torah obligation, but the structure of hearing it in a Torah reading in public on the Shabbos before Purim is Rabbinic].
In terms of a Nafka Mina, I haven't heard of any.
