About Rashi's Shita below, who hold that by pouring four cups into one, Yedei Cheirus Yotso, Yedei Arba Kosos Lo Yotso, is more difficult to understand than the shita of other Rishonim who explain the Gemara to mean that he drank 4 cuts one after the other, not al Haseder.
Does Rashi mean that he drank the entire kos in one shot ? Or maybe he means that he drank it al Haseder , 1/4 at a time, and therefore accomplished the ideas of Cherus and Hagada properly ?
The term the Gemara uses, "b'Bas Achas" certainly implies the manner in which he drank the cups of wine -- that is, all at once. If the Gemara (according to Rashi) was saying that he merely poured them all into one cup and drank a fourth of that large cup at a time in the proper order, then the Gemara would have said "b'Kos Echad," I should think.