Are the four cups of wine that we drink at the Pesach Seder m'akev (bar) each other. That is to say, without all four cups, do the cups a person does drink count for anything?
However one will ask, "l'mai nafkah minah". That is, how could the four cups be "m'akev" each other and what difference does it make?
Obviously a practical nafka minah would be: If one does not have four cups, is there any point in drinking three?
Seemingly there could be another. It says in the Mishnah in the beginning of Arvei Pesachim that if someone doesn't have four cups they take from the "tamchui" (take from Tzedeka). If the person is only able to get two cups from Tzedeka and can't get all four, does he still take? Perhaps only when he'll get four and be able to fulfill the mitzvah in it's fullest would he take from tzedeka.
As always provide sources and perhaps other nafkah minei for this question.