One of the unique things that we do at the Seder is to recline when we eat matzah and drink the four cups of wine. In truth the Rambam says that it is praiseworthy to recline at the rest of the meal as well. The Gemara in Pesachim 108a says that one needs to recline only while drinking two of the four cups of wine. However, the Gemara is unsure which two of the four one must recline for. The debate is whether it is the first two or the last two. The Gemara says that since we are unsure today we must recline when drinking all four cups of wine.
Tosafos there and the Rosh state that if one does not recline when obligated to, i.e. while eating the matzah or dinking the cups of wine, he must eat or drink again while leaning. Tosafos says that since we are not certain during which cups we are obligated to recline while drinking, if one is unsure if he reclined during one of them it is a safek as to whether he has to drink that cup again.
The Ran asks why the Gemara concludes that since we are unsure which two of the four we must recline for, that we must recline for all four. Drinking the four cups of wine is a mitzvah m’d’Rabanan and the obligation to recline while drinking them is d’Rabanan as well. Since this is a safek d’Rabanan we should be lenient and not require one to recline while drinking any of the cups of wine. He suggests two answers. First that it is not difficult to recline, so even though it is a safek d’Rabanan we fulfill both opinions. Second, the Ran suggests that we cannot choose which of the two to perform. Therefore we do both opinions, because if we would not do either opinion we would be mevatel the entire mitzvah of reclining during the four cups. We do not apply the halachah of safek d’Rabanan l’kulah if it will result in abolishing the mitzvah entirely.
The acharonim point out a contradiction, based on the Ran in Maseches Megillah. There the Ran is addressing the halachah that a city that was walled from the times of Yehoshua ben Nun reads the Megillah on the fifteenth of Adar, while every other city reads the Megillah on the fourteenth of Adar. The question is regarding a city that we are unsure whether it was walled from then or not. Which day should its inhabitants read the Megillah? The Ran says that since it is a safek d’Rabanan they should not have to read the Megillah on either day. However this would essentially be mevatel the entire mitzvah for them. Therefore, they should read the Megillah on the fourteenth day of Adar and not on the fifteenth.
The Mishnah L’melech asks that this Ran seems to contradict our Ran in Pesachim. In Pesachim the second answer of the Ran suggested that since it was a safek d’Rabanan we should perform both opinions so as not to be mevatel the entire mitzvah, while in Megillah the Ran says that the inhabitants of the city only have to read the Megillah on one of the days.
The Mishnah L’melech understands that the reason that the Ran in Megillah says to read the Megillah on the fourteenth is because the first opportunity one has to perform a mitzvah he must do it. Therefore, they should read the Megillah on the fourteenth since that day precedes the fifteenth. Based on this, the Mishnah L’melech says, the Ran should conclude that one should drink only the first two cups while reclining since that mitzvah comes first.