I don’t know if it’s fair to be madayeik in 2 works of the Netziv in Ha’amek She’eilah – I’ll write and you can decide. Rashi on the first Mishna in Arvei Pesachim writes that the 4 kosos correspond to the 4 leshonos of geulah in our parsha. (Rashi later on daf 108 interestingly gives a different reason and says the 4 cups correspond to 4 times cups are mentioned in the dream of the Sar haMashkim.) The Mordechai asks: why 4 cups and not 4 matzos? (Not a question according to the Rashi on 108). Mordechai answers that cups are mentioned in pesukim that relate to geulah, e.g. kos yeshu’os esa, so we use cups of wine. The Netziv just slightly rephrases the Mordechai’s question. The way he puts it is why 4 cups and not 4 matzos or 4 pieces of meat u’k’domeh. In other words, if all we are interested in is representing the number 4, who cares what we use or how we do it. That’s a very broad way to read the Mordechai. You could read the question more narrowly: why did Chazal institute a new mitzvah of 4 kosos instead of building the representation of number 4 into an existing mitzvah, e.g. matzah. The Netziv’s reading is more like a derush/philosophical/ta’amei hamitzvos question; the more narrow reading is a “lomdish” question regarding the structure of the din.
Just for the record, the Netziv answers that the idea of the 4 leshonos is that geulah is a gradual process. You don’t go from being a slave to a free man overnight. Even if the shackles are off, there is a psychological adjustment, a social adjustment that has to take place. Chazal used 4 cups to represent this gradual shift because when a person drinks, there is a gradual change that happens as the person drifts closer to (or deeper into) inebriation.
R’ Tzadok haKohen (Pri Tzadik, Pesach 5) adds an extra twist to the question and formulates it like this: the 4 kosos are a din derabbanan; matzah is a din d’oraysa. Why would Chazal incorporate the representation of 4 into a derabbanan when they could have incorporated it into a d’oraysa?
R’ Tzadok (k’darko) sees the difference between d’oraysa and derabbanan as not just a technical distinction, but as representative of the difference between what is ingrained in a person’s neshoma vs. what can be achieved through avodah. The 4 steps of geulah are 4 stages in defining who we are as Jews. The pinnacle of all these stages is the permanent banishment of the yetzer ha’ra from within. We got to that level at mattan Torah, but lost it almost immediately – it did not become permanently ingrained in us. 3 of the 4 leshonos of geulah were, but we missed the final step. Therefore, when it comes to matzah d’oraysa, reflecting what is innately part of us, we only take 3, but not 4 matzos. When it comes to kosos, we take 4 as a symbol of what we hope to achieve by dint of our efforts, through torah sheba’al peh. Matzah, 3, is about who I am now; kosos, 4, is about where I am going and what I want to become.