Hacham Ovadia Yosef, in his Hazon Ovadia (p. 124; listen to audio recording for precise citation), writes that one should endeavor to complete the recitation of Hallel at the Seder, and to drink the fourth cup of wine, before Hasot (midnight as defined by Halacha). Nevertheless, he adds, if one was unable to complete Hallel by Hasot, he may recite the concluding Beracha of "Yehalelucha." Hacham Ovadia cites the ruling of the Tosafot (Medieval French and German commentators) in Masechet Megila (21) that it is only regarding the Torah obligation of Masa that Halacha insists on performing the Misva before Hasot. Hallel is required only Mi’de’rabbanan (by force of Rabbinic enactment), and thus although one should certainly try to complete the Hallel recitation before Hasot, it may be recited with a Beracha even afterward.
One should ensure to drink a Rebi’it (81 grams) of wine when drinking the fourth cup of wine at the Seder. Although the obligation of four cups can be fulfilled by drinking just a mouthful of wine, one should try to drink a Rebi’it for the fourth cup so that he can recite a Beracha Aharona without any Halachic uncertainty. This Halacha is mentioned by the Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327). (It should be noted that if one drank a Rebi’it for the third cup of wine, after Birkat Hamazon, then, strictly speaking, he does not have to drink a Rebi’it for the fourth cup, as he is in any event required to recite a Beracha Aharona.)
One must drink the four cups of wine at the Seder "Be’heseba," meaning, while reclining. If one mistakenly recited the fourth cup of wine without reclining, and when he realizes his mistake there is still some wine remaining in his cup, then he should add more wine to the cup and then drink while reclining. In such a case, he does not recite a Beracha when he drinks the second time. If the cup was empty by the time he realized his mistake, then he must fill the cup anew and drink while reclining. According to the ruling of the Shulhan Aruch, in such a case one should recite a new Beracha of "Boreh Peri Ha’gefen" over the new cup of wine, since he had already taken his mind off drinking when he completed the original cup. Although Halacha follows this view, some authorities maintained that a Beracha should not be recited in this case, and thus if one wishes, he can think the Beracha in his mind, rather than actually recite it, in order to satisfy all opinions. Strictly speaking, however, one recites a Beracha when drinking a new cup of wine in this situation.
Summary: One should endeavor to complete Hallel at the Seder before Hasot, but nevertheless, if Hasot had already passed, the Beracha of "Yehalelucha" is still recited. One should drink a Rebi’it (81 grams) of wine for the fourth cup of wine at the Seder. If one mistakenly did not recline while drinking the fourth cup of wine, and some wine remains in the cup, he should add some wine and then drink, without a Beracha. If no wine remains in the cup, he must pour again and drink a new cup, with a new Beracha.
