The Sages stated (Pesaḥim 108b) that in order to fulfill the mitzva of the four cups properly, one must dilute the wine with water, because otherwise it will be too strong and heavy and will cause intoxication. Though the alcohol in such wine gives one pleasure, the mitzva is to drink the wine in the manner of free people, that is, like wealthy people who have control of their own time and permit themselves to drink the best wine, making sure it is diluted properly so that one can enjoy it without getting drunk. Today’s wine is not as hard and strong, and there is no obligation to dilute it. Only one for whom diluting the wine improves its taste and effects has a mitzva to dilute it. (See MB 472:29; some have the custom of diluting the wine to enhance the mitzva.)
Instead, nowadays we enhance the expression of our freedom by purchasing fine wine that is strong enough to intoxicate, because such wine causes a sense of joy, liberation, and freedom. However, one must be careful not to use a wine so strong that it disrupts one’s ability to concentrate on the Hagada and to fulfill the mitzvot of the Seder night. One must therefore drink four cups of wine in a manner that brings joy and does not cause drowsiness or intoxication. A person for whom drinking a full cup of wine is hard or might hinder him from fulfilling the mitzvot of the Seder properly should mix his wine with water, grape juice, or both, as long as water is not the majority. This will allow him to drink wine that intoxicates, on one hand, but on the other hand, will bring him to a state of joy in which he can properly fulfill all the mitzvot of the Seder night. One may also use wine with very low alcohol content.
Even though, be-di’avad, one can fulfill the obligation with grape juice, one must know that when the Sages instituted the four cups, they did not imagine that one would fulfill this mitzva with grape juice, as in their day there was no way to preserve the juice without it turning into vinegar. Even one who did not enjoy the taste of wine or who got a headache from drinking wine was obligated to drink four cups. In fact, the Gemara recounts that R. Yehuda b. Ilai had to wrap his head in a kerchief from Pesaḥ until Shavu’ot due to a headache caused by drinking four cups of wine. Only one who wine caused to become ill and bedridden was exempt from the mitzva (Nedarim 49b; SA 472:10; MB ad loc. 35). The Sages intended for us to recite the Hagada and tell the Exodus story while drinking wine that enhances enjoyment. One who uses grape juice does not fulfill the mitzva in the way the Sages enacted it. However, now that grape juice is available, one who suffers from the effects of wine – for example, it gives him a headache – may fulfill his obligation using grape juice.
Women are also commanded to drink four cups of wine, just as they are commanded to fulfill all of the mitzvot of the Seder night (SA 472:14). The mitzva is to drink wine that enhances joy.
There is a mitzva to embellish this obligation and choose a fine wine that brings joy to use for the four cups. In addition, red wine is considered preferable. However, fine white wine is preferred over red wine that is not as fine. Be-di’avad, any wine is adequate, even cheap white wine (SA 472:11).