Passover- The Third and Fourth Cups of Wine at the Seder
Towards the end of the Seder we recite Hallel over the fourth and final cup of wine. The practice of the Sephardim is not to recite the Beracha of "Borei Peri Ha'gefen" before drinking the fourth cup. We instead rely on the Beracha recited before drinking the third cup of wine, (before Birkat Ha'mazon). Therefore when one recites "Borei Peri Ha'gefen", before drinking the third cup of wine, he must have in mind that it should apply to the fourth cup that he will drink after Hallel as well.
Regarding Beracha Acharona:
Does one recite a “al ha'gefen Ve'al Peri Ha'gefen”, in all situations?
According to the Halacha, one recites a Beracha Acharona only if he drinks a quantity of a Revi'it, (approximately 3 oz.) in one shot. However, one fulfills his obligation to drink each cup of wine at the Seder by drinking a majority of a Revi'it, (approximately 1.6 oz.). It is therefore possible for somebody to fulfill his obligation of drinking at the Seder without becoming obligated to recite a Beracha Acharona. Hence, one recites a Beracha Acharona after drinking the fourth cup of wine, but only if he drinks at least 3 oz. in one shot during either the 3rd or 4th cup. (The amounts drunk for the third and fourth cups do not combine to constitute a Revi'it with respect to the requirement to recite a Beracha Acharona.)
One should remember to drink the third and fourth cups while reclining on his side.
Chacham Ovadia Yosef writes (listen to audio for precise citation) that one should complete the Hallel recitation and the drinking of the fourth cup of wine at the Seder before Chatzot (midnight as defined by Halacha). Nevertheless, if one did not complete Hallel by Chatzot, he may nevertheless recite the final Beracha of the Hallel ("Yehalelucha") and should proceed with the rest of the Seder as usual.
Summary: Our practice is not to recite a Beracha over the fourth cup of wine at the Seder, and to instead rely on the Beracha recited over the third cup; one should have this in mind when reciting this Beracha. One recites a Beracha Acharona after the fourth cup of wine, but only if he drank at least 3 oz. either in the 3rd or 4th cup in one shot. One should try to complete Hallel and drink the fourth cup before Chatzot; if one did not complete Hallel before Chatzot, he nevertheless continues with the rest of the Seder as usual.