In your insights to daf 51, #3, answer b, you quote Rabeinu Yonah and his worry about 'zugos'. On Pesach, drinking he cannot concentrate properly because of his worry of Mazikin, so therefore he cannot say Birkas ha'Mazon on behalf of others. Even though the Meiri (here) seems to say that today, we're not concerned with zugos, how did Raeinu Yonah (and others in those times when there was a concern about zugos and the resulting harm from Mazikin) deal with 4 cups on Pesach?
Sincerely,
Jeff Ram
The Gemara there in Pesachim (109b) deals with your question and answers that the Kos Shel Berachah (that is, the third cup of wine) is not included in the count. Rav Nachman there answers that since Pesach night is "Leil Shimurim," a night of Divine protection, there is no concern for Mazikin. Other answers are given there in the Gemara.
Kollel Iyun Hadaf