If one only has 4 cups of wine and 4 people are at the Seder table, is it better that each person drinks 1 cup or 1 person drinks all four?
This is an interesting question, and there are two main angles that must be raised.
On the one hand, there is room to question whether each cup is considered a mitzvah in itself, independent of the other cups. Mishnah Berurah writes (483:1) that one who has three cups should drink them, yet there is room to argue that he means that one should bench with a cup, and make kiddush on a cup, rather than implying that there is a mitzvah in each cup itself. Rav Chaim Kanievsky is quoted (Kol Torah 38, p. 67) as stating that there is no mitzvah in each independent cup, and this stands to reason, for the entire enactment was four cups. Yet, Rashash (Pesachim 109) derives from the Gemara that each cup is considered a mitzvah in itself.
On the other hand, there is room to debate whether or not one person can be motzi others the mitzvah of the four cups. This is stated as a safek by Tosafos, and see Chazon Ish (Orach Chaim 39:1-3), who explains the doubt based on the issue of shomea ke’oneh. Other rishonim, however, write that there is a mitzvah in the drinking itself, which of course one person cannot perform for another.
It would appear, based on the above, that it is better for one person to drink all four cups. This way, at least one person fulfills the mitzvah of the cups, and it is possible that all fulfill the mitzvah, based on the doubt expressed by Tosafos. If all four drink one cup, nobody fulfills the full mitzvah, and it is possible that nobody fulfills any mitvzah, if the mitzvah is limited to all four cups.
However, it is noteworthy that according to Mikraei Kodesh (II, no. 30), the two questions are related. He explains that there are two mitzvos involved in the four cups: one in drinking them, and the other in reciting the blessings and the haggadah on the cups. For the mitzvah of drinking, he writes that the mitzvah requires four cups, and less than this does not constitute any mitzvah. For the recitation, each one is a mitzvah.
According to this, if one person drinks four cups, the person drinking fulfills both mitzvos of the cups, whereas the others would possible fulfill only one of the mitzvos, whereas if all drink one cup, all would fulfill some part of the mitzvah in reciting one blessing on the cup. Nonetheless, the former option would appear preferable.