Our main focus in our answer will be on Ashkenazi men. (Many Ashkenazi women do not recline while drinking, and a woman who does recline has more room for leniency if she forgot.[1] We will present the simpler instructions for Sephardim at the end.)
The gemara[2] says that, in principle, one must recline only for two of the four cups of wine. However, since it could not conclude whether that means the first two or the last two, the ruling is to recline for all four. The Rosh[3] stipulates that if one was supposed to recline (while eating matza or while drinking wine) and did not, he did not fulfill the mitzva properly and must eat or drink again. (One does not repeat the section of the Seder related to the cup, just the drinking.) However, the Rosh wonders whether the repetition is appropriate for the third or fourth cup. There is an obligation to drink four cups, but one should not drink more than that in a manner that gives the appearance that one is drinking as a formal part of the Seder. Since one is not allowed to drink wine after the third cup (obviously, excluding the fourth cup) or after the fourth cup,[4] drinking more suggests that one is adding a fifth official cup of wine. This makes repeating potentially problematic. On the other hand, the Rosh reasons that since one did not fulfill the mitzva properly if he drank without reclining, that cup did not count, and he can and should drink another. The Rosh provides no resolution to the quandary.
The halachic implications are even more complex nowadays for Ashkenazim. The original reason to eat and drink while reclined was that it demonstrates that we are liberated people, as that is how important people once ate at leisure. The Ra’avya[5] makes the argument that since, nowadays, important people eat sitting erect, reclining no longer accomplishes its intended function, and it is therefore no longer necessary. The Rama factors this argument into his rulings. He uses it not only to explain the minhag of women in his time not to recline,[6] but also as a reason not to require a man who forgot to recline to repeat the eating[7] or drinking. However, the Rama concludes that regarding wine, one should implement the Rosh’s distinction in the following manner.[8] For the first two cups, where there is no halachic downside to drinking more, he should repeat. For the latter cups, where there is a problem of appearing as though one is adding cups, one should rely on the Ra’avya that drinking erect is sufficient and avoid further drinking, which would be unnecessary and improper according to his view.
The Magen Avraham[9] found an issue even with repeating the first cups. As we discussed above, drinking a cup of wine that one normally should not be drinking makes it look like a mandatory cup. Logically, making an additional beracha on the wine also makes it look formal. The Magen Avraham points out that even though one may drink wine between the first two cups, we are no longer in the practice of doing so.[10] Therefore, one’s intention with the beracha over the first cup is for that cup alone. Repeating the cup would thus entail reciting another beracha, which makes it look like a problematic additional cup. The Magen Avraham argues that we should thus rely on the Ra’avya rather than repeat the first cup.
Regarding forgetting to recline for the second cup, if one had any thoughts of drinking wine during the meal, another beracha would not be necessary, and it would be preferable to repeat the cup.[11] The Magen Avraham says that it is best, when preparing to drink the first cups, to have in mind that the beracha should apply to any further drinking.[12]
The Mishna Berura[13] seems to endorse the Magen Avraham’s approach. Given that the Magen Avraham’s criteria are subjective and hard to determine, and since the matter is a Rabbinic one with a few mitigating doubts, one can resolve ambiguity on the side of leniency (i.e., not to repeat). (If and when one does repeat, drinking grape juice is a wise choice.)
For Sephardi men, the situation is simpler. The Shulchan Aruch[14] does not rely on the Ra’avya on this subject. Matters are further simplified by the fact that according to the rulings accepted by Sephardim, the berachot on the first and third cups exempt the berachot on the second and fourth, respectively. Accordingly, a Sephardi man should always repeat the cup if he forgot to recline while drinking.