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.