In regards to Tosfos DH Lo
What is the sofek of Tosfos exactly? Why is it that he doesn't find it so simple that for each person to fulfill the mitzvah of arba cosos he must drink every cos? Why does he seem to compare (all arba cosos) to that of Kiddush the whole year that one may say the kiddush, drink the wine and everyone is yotzee with him.
I had seen brought down from the Gaonim that the arba cosos of the Pesach Seder are a "chovo" and impossible to not drink them (See here:http://www.hebrewbooks.org/pdfpager.aspx?req=22172&st=&pgnum=89 brought down in Gilonei HaShas from R' Yosef Engel)
Yehoshua, Yerushalayim, Eretz Yisrael
Sholom Rav
I think that Tosfos' opening words answer your question. The fact that the Mishnah writes 'They should not give him less than four cups of wine' implies that they give him four cups, but not four cups for each member of his family.
Note, that Tosfos does write that 'it implies slightly'. They clearly concede that the Tana could mean four cups per family member, but this not the strict implication of their statement. In any event, that is Tosfos' starting point.
Be'Virchas Kol Tuv
Eliezer Chrysler