Le-khatḥila, it is best to enhance the mitzva by drinking all the wine in the cup. Even if the cup is large and contains much more than a revi’it (75 ml), he should, le-khatḥila, drink all of the wine in the cup. If he does not want to, he should at least try to drink most of the wine in the cup. At the very least, one must drink most of a revi’it and a “melo lugmav.” In other words, there are two conditions: he must drink most of the wine that is required to be in the cup, i.e., most of a revi’it (38 ml), and this amount must be at least melo lugmav – enough wine to fill the drinker’s mouth with one cheek inflated. This is the amount of wine that settles one’s mind. For someone with a normal size mouth, melo lugmav a bit more than most of a revi’it. For one with a large mouth, melo lugmav is closer to a revi’it. A thirteen-year-old, whose mouth is small and whose melo lugmav is less than half a revi’it must drink most of a revi’it in order to fulfill the first condition (SA 472:9; MB ad loc. 30; BHL ad loc. s.v. “ve-yishteh”).
Children who have reached the age of mitzva education are given four cups of wine (SA 472:15). The age of education for this mitzva is when the child understands the meaning of the things that are said while the cups are filled – kiddush, the Hagada, Birkat Ha-mazon, and Hallel (SAH 472:25) – generally around age five or six. There is no need for them to drink most of a revi’it; melo lugmav suffices (MB 472:47).
One must drink most of the wine in the cup “at once.” If one drinks half of the necessary amount, and then, after a long pause, drinks the second half, this is not considered a single “drinking,” and thus he did not drink the necessary amount of wine to fulfill drinking one of the four cups. However, leading Rishonim and Aḥaronim are divided over precisely how quickly one must drink the requisite amount of wine in order for it to be considered “at once.” According to Rambam, since people are accustomed to drink continuously, albeit with short breaks to breathe or swallow, one is only considered to have drunk “at once” if he drank continuously, as one would normally drink a revi’it. In other words, the amount of time it takes to drink the quantity of wine that one must drink from every cup is the amount of time it normally takes to drink an entire revi’it. If it takes one longer to drink most of a revi’it than it normally takes people to drink a revi’it, he has not fulfilled the mitzva. Raavad, on the other hand, maintains that as one managed to drink the requisite amount of wine within the time it takes to eat a half a loaf of bread (“shi’ur akhilat pras”; c. seven minutes), it is still considered one “drinking”.
To summarize, le-khatḥila one should follow the stringent ruling of Rambam and drink the quantity of wine within which one fulfills his obligation continuously, taking short breaks to breathe and swallow. Be-di’avad, if one drinks it within a shi’ur akhilat pras (c. seven minutes), he fulfilled his obligation, since the rule of four cups is of rabbinic origin, and the halakha therefore follows the lenient position. Nonetheless, some practice stringency.
Although halakha follows the widely accepted opinion, in this context I stressed that following the Ḥazon Ish shi’ur enhances the mitzva, since more wine enhances the mitzva of rejoicing, and a revi’it is a minimum. Those who enhance the mitzva always drank more. Thus, to satisfy unquestionably all opinions one should le-khatḥila use a cup that contains 150 ml and drink most of it. This satisfies the higher melo lugmav requirement, and since he drank most of the cup, he fulfilled his obligation according to Ramban as well. However, if it is difficult for him to drink an entire 150 ml “Ḥazon Ish shi’ur” cup, he ends up losing the enhancement of drinking an entire cup. Although we have stated that one may fulfill his obligation le-khatḥila by using the standard shi’ur, one who wishes to enhance the mitzva should first satisfy all opinions, including the Ḥazon Ish shi’ur, and only then undertake to drink an entire cup. If he enjoys drinking wine, the greatest enhancement is to drink a full 150 ml cup; if he is worried that he might become intoxicated, he may mix grape juice into the wine.
. More broadly, “shi’ur akhilat pras” is a unit of time within which all eating is treated as a single act. There is a dispute as to whether or not this time period applies to drinking as well. According to Rambam, since one drinks faster than he eats, the measurement of time for drinking is different than that of eating, and only uninterrupted drinking is considered as one unit. According to Raavad, the measurement of time for drinking is the same as for eating, and as long as one drank within a “shi’ur akhilat pras,” it is considered a single “drinking.”
Le-khatḥila, one must drink uninterruptedly, as per Rambam, and if he drank the minimum amount within the time of akhilat pras, he has not fulfilled his obligation according to Rambam, only according to Raavad. But since we rule leniently when there is uncertainty pertaining to rabbinic law, one would not have to drink again. This is what AHS 472:13 and Ḥazon Ovadia §12 state. Moreover, according to some, Rambam only stated his opinion with regard to a forbidden drink, where one incurs lashes if he drinks continuously. But regarding berakhot over consumption of food or drink, Rambam would concur that anything drunk within a shi’ur akhilat pras is combined together, since one derives pleasure from drinking even at this slower pace (Knesset Ha-gedola, SAH, and Ḥatam Sofer). Perhaps according to these authorities, Rambam would concur that one who drinks a cup of wine within a shi’ur akhilat pras would fulfill his obligation. Nevertheless, we find in several places that where it is easy to fulfill one’s obligation according to all opinions, one should be stringent even with regard to a disputed rabbinic law. Therefore, MA and SAH 472:20 and MB 472:34 state that if one drank the second cup only within the time of akhilat pras, he should drink it again. If it was the third or fourth cup, though, he should not drink again, since it would look like he was adding to the four cups. Rather, in this case he should rely on the opinion of Raavad that he has fulfilled his obligation even if he interrupted his drinking, as long as he finished drinking within a shi’ur akhilat pras. (Regarding the first cup, see MB 472:21.) According to those who follow SA that one may drink after the third and fourth cups, one should re-drink any of the four cups in question, as Ben Ish Ḥai (Tzav 29) states. This is why I wrote that some are stringent in this matter, despite the fact that this is a double uncertainty about a rabbinic law: according to Raavad, the obligation has certainly been fulfilled, and possibly even according to Rambam as well. (Perhaps there is a tendency toward stringency, beyond what is required by halakhic principles, with respect to the mitzva of rejoicing.)
See section 25 below and note 22 ad loc. for the differing opinions regarding the length of shi’ur akhilat pras. The average time is around six to seven minutes, but le-khatḥila it is best to adopt the more stringent amount of four minutes, and under extenuating circumstances one may be lenient and count up to nine minutes.