Query: May one drink the four cups one after another?

Answer: From the statement of R. Yochanan that if one heard Hallel in the synagogue (and he intented to fulfil his obligation to recite it on the night of Pesach), he has fulfilled his obligation of Hallel and he need not repeat it at home during the Seder; this shows that if he drank all of the four cups one after another, he has also fulfilled his obligation.

Query: May one drink the cups by sipping them several times?

Answer: Did they not say that he should drink but not become drunk? If he drinks the cups by sipping them, he will anyway not become drunk (so he cannot fulfill his obligation by drinking in this way).