(Please forgive the delay in response. Technical problems prevented the mailing of a number of responses.)

1) This question is not the same as the famous Ran, but the answer of the Ran can also answer your question, because the Ran writes that since, in this case, there is not a big effort involved in leaning for all four cups, we do so "l'Ravcha d'Milta" -- i.e. we lean even though it is not Halachically essential. This idea also answers why we do not say "d'Avad k'Mar Avad," because since it is easy to lean for all four cups we say that it is preferable to do so rather than to choose only one of the opinions in the Gemara. (Clearly the latter logic does not apply concerning the Minchah/Ma'ariv issue in Berachos 27a.)

2) The Maharam Chalavah here adds to this answer the phrase, "Miheyos Tov Al Tikrei Ra" (see Bava Kama 81b) -- "If one can do something good, then do not do something bad." In other words, since it is easy to lean for all four cups, one should cover all the different opinions and do so.

(Rashi in Berachos 30a (DH Miheyos) writes that even though it might be permitted according to the Halachah to do otherwise, it is still better to behave in the preferable way.)

3) The Shiltei Giborim on the Mordechai in Avodah Zarah (#845:10, cited by Mesivta edition of the Gemara) gives an explanation for our Gemara which can also answer your question. He writes that our Gemara does not say explicitly who says that one must lean for the first two cups and who says for the last two, but rather we are in doubt about who said what. Therefore, we have to fulfil all the opinions. If one would lean only for two of the cups, one would not know exactly whose opinion he is relying on. This is not the same as the dispute about Ma'ariv, where one knows for sure who said what, and if one does according to one of those opinions, he is certain that he is following closely one specific opinion.

4) I looked around some more among the Mefarshim and found that your question is asked by the Steipler Gaon zt'l in Kehilos Yakov, Berachos #1. Baruch she'Kivanta!

5) In 1:3, the Kehilos Yakov gives the answer that I wrote above in the name of the Ran, that since it is not difficult to lean four times, one should do so.

5) The Kehilos Yakov gives another answer based on the Me'iri in Berachos (27a) who cites an opinion that if a person once recites Minchah between Plag ha'Minchah and sunset, he may never again recite Ma'ariv at that time because he has accepted this time as being daytime. The Me'iri wrties that he does not agree with this opinion at all. He writes that in his opinion, on the same day one should not recite both Minchah and Ma'ariv in the same time period. However, on different days there is no problem with sometimes reciting Minchah at this time, and on other days reciting Ma'ariv then. The Me'iri writes that this is not considered "Tarti d'Satri" -- two practices which contradict each other -- because for a Mitzvah d'Rabanan such as Tefilah, one is not concerned about such a problem. The Me'iri writes that this is similar to the case of the two paths (Pesachim 10a), one of which is Tamei and the other is Tahor. One person walked on the Tamei path and one walked on the Tahor path and we do not know who walked on which. If the two people come simultaneously to ask what their status is, then we have to tell them that they are both Tamei. However, if they come one after the other, we tell each one that he is Tahor.

6) The Kehilos Yakov answers your question based on the Me'iri. He asserts that the Mitzvos of the Seder night are comparable to the case where two people walked on the two paths and came to ask simultaneously in Beis Din what their status is. We have to tell them both that they are Tamei. Similarly, all of the Dinim of the Seder night are here in front of us at one time, so we cannot decide that one needs to lean only for two of the cups, and for the other two he does not.

Dovid Bloom