Before answering which cup of the Passover Seder meal it was, we must establish there were "four cups" in the first place. And this is precluding the discussion that the Last Supper even was the Passover meal.
Historical accounts and archeological evidence points to the Seder not existing as we know it today before 70 CE, the destruction of the temple and the Diaspora of the Jews. Second Temple Jewish writers such as Josephus seem to be unaware of it.
...practically everything preserved in the early rabbinic traditions concerning the Passover Seder brings us back to the time immediately following the Roman destruction of the Temple in 70 C.E. It’s not that rabbinic literature cannot be trusted to tell us about history in the first century of the Common Era. It’s that rabbinic literature—in the case of the Seder—does not even claim to be telling us how the Seder was performed before the destruction of the Temple. 1
In an excellent summary on the (Origins of the Seder) 2, evidence is given of possible Greek banquet customs having an influence. On the cups:
According to the Mishnah (10:1), a person must drink four cups of wine at the Seder. The Greeks too drank many cups of wine at the symposium. Antiphanes (4th century B.C.E.) said that one should honor the gods to the extent of three cups of wine (Stein, p. 17).
So we see that even if there was a form of the Seder at the time of the Last Supper, there are no assurances of what the early traditions were. There could have been one, two, three, four or even more cups!
So what DO we know of the Last Supper?
Presumably, Jesus and his disciples would have visited the Temple to slaughter their Passover sacrifice. Then they would have consumed it along with unleavened bread and bitter herbs, as required by the Book of Exodus. And presumably they would have engaged in conversation pertinent to the occasion. But we cannot know for sure. 1
Exodus 12 gives us the elements of the lamb, the unleavened bread and the bitter herbs, so it is reasonable to conclude a Jewish Passover meal would include them somehow. Wine also would be the most common drink.
Given that the cups of wine are not part of the original elements of the meal and no evidence of them exist before the Diaspora, we cannot make any claims or conclusions on how the cup that Jesus passed would compare to our modern Seder traditions, even if it was the same meal.