Do we look at the 4 cups of wine that we drank by the Pesach Seder as "one mitzvah" and therefore all four cups are m'akev ("hold back") each other. That is to say without all 4 cups the person is lacking something in the fulfillment of the mitzvah.

However one will ask, "l'mai nafkah minah". How could the 4 cups be "m'akev" each other and what difference does it make?

While I haven't come up with a good nafkah minah yet, seemingly there could be one. If says in the Mishnah in the beginning of Arvei Pesachim that if someone doesn't have 4 cups they take from the "tamchui" (take from Tzedeka.) If the person is only able to get 2 cups for Tzedeka and can't get all 4, does he still take? Perhaps it's only when he'll get 4 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 the this question.