Time to start transitioning from Purim to Pesach, and what better way to do that than with a question of the Mishne laMelech. The Ran in Megillah says that a city which is safeik mukaf chomah / safeik not mukaf chomah and therefore in doubt whether to read megillah on the 14th or the 15th should read on the 14th with a bracha. Ran writes that you can't say safeik derabbanan l'kula on each individual day (and parenthetically, you see from this Ran that the chiyuv kri'ah which is m'divrei kabbalah is treated like any other safeik derabbanan) because then you would not end up reading on either day and then entire mitzvah would be uprooted. Therefore, since the first opportunity to read is the 14th, Ran says to read on that day, say a bracha because this is the day that most of the world reads on (rov), and then once the 15th comes you can assume that you already fulfilled the mitzvah the day before and apply safeik derabbanan l'kula.
There is another Ran that we discussed about a year ago (see here, here, here) that the M.l"M. suggests contradicts this Ran. The gemara in Pesachim tells us that only two of the four kosos we drink on Pesach night require haseiba but leaves unresolved as to whether it is the first two or the last two kosos. Therefore, concludes the gemara, we must do haseiba for all four kosos. The Ran asks: why not apply the rule of safeik derabban l'kula to this issue and not require haseiba at all? Ran answers that we cannot apply the rule of sfeika derabbanan where it would eliminate the mitzvah of haseiba entirely. Shouldn't there be more to the Ran than this? Based on the Ran's own reasoning in Megillah, shouldn't the logical conclusion to the gemara's question be to do haseiba on the first two cups, as that is the first opportunity to do the mitzvah of haseiba, and then rely on safeik derabbanan l'kula for the last two cups? Why does the Ran employ this line of reasoning with respect to megillah but not with respect to the mitzvah of 4 kosos?