On Pesach night we drink 4 cups of wine in remembrance of the 4 expressions of geulah, redemption, used in our parsha (Rashbam). These cups must be drunk b’haseiba, while reclining, but there is some ambiguity as to which cups this is required for. The gemara (Pes 108) quotes two versions of a statement of R’ Nachman, one stating that the first 2 cups require haseiba, and one stating that the last 2 cups require haseiba. Since the tradition is unclear, we recline for all 4 cups.
The Ran asks why we do not apply the rule of safeik derabbanan l’kula to this case – when in doubt about a Rabbinic law, one has the right to be lenient, therefore with respect to any of the cups since there is a doubt if haseiba is required at all, we should assume it isn’t. The Ran offers two answers: 1) Since it is not burdensome (tircha) to recline, why not do so if one is able to (based on Rashi); 2) If one is lenient regarding haseiba for both the first and last cups one has completely negated the mitzvah, which cannot be done. Based on either reason, if one inadvertently drank a cup without haseiba one would not have to redrink – in such a case the drinking of extra cups does become a burden, and if one misses haseiba by accident it does not change the ideal halachic requirement of ideally drinking b’haseiba, which was the concern of the second reason.
The Bais HaLevi (III:1:9) notes that many Rishonim (e.g. Tosfos, Rosh) disagree with this conclusion and hold that if one drank without haseiba, one would indeed be required to redrink the cup, whichever one it is. Why do this Rishonim not apply the principle of safeik derabbanan l’kula to this case?