Many people ask whether grape juice may be used instead of wine for the Misva of "Arba Kosot" – drinking four cups of wine at the Seder. This is, indeed, an important question with which the Halachic authorities have grappled, both in regard to this Misva as well as in regard to the Misva of Kiddush.
When it comes to Kiddush, the Rambam (Rav Moshe Maimonides, Spain-Egypt, 1135-1204) ruled that wine which is Mebushal – meaning, it has been brought to a boil – may not be used. The Shulhan Aruch (Orah Haim 472) does not follow this view of the Rambam, but according to some authorities, it is preferable not to use Mebushal wine for Kiddush. The grape juice which is commonly sold in stores today is Mebushal (as it has undergone pasteurization), so this is one disadvantage of using grape juice for Kiddush. It should be noted that the Shulhan Aruch permits using for Kiddush freshly-squeezed grape juice produced on Friday, so if somebody squeezes grapes to produce grape juice before Shabbat, which is obviously not Mebushal, he avoids this disadvantage.
The second Halachic disadvantage of grape juice is that it fails to meet the standard mentioned by several Halachic authorities (Magen Abraham, and Hid"a in Birkeh Yosef) who maintained that one should preferably use aged wine for Kiddush. This refers to wine which was contained in a vat for at least forty days. Grape juice never even begins the fermenting process, and thus certainly does not meet this preferred criterion.
Thus, although from a strict Halachic standpoint grape juice may be used for Kiddush on Shabbat, it is preferable to use wine. Ideally, one should use non-Mebushal, aged wine. The next level would be aged wine that is Mebushal. The third level would be non-Mebushal grape juice, and the fourth level would be Mebushal grape juice.
When it comes to the four cups at the Seder, there is an additional factor that must be taken into consideration. Namely, the wine drunk at the Seder must have the quality of "Mesame’ah" – it brings a person joy. The Halachic authorities address the question of whether grape juice, which has no intoxicating effect, meets this requirement. Hacham Ovadia Yosef draws proof from the comments of the Ran (Rabbenu Nissim of Gerona, Spain, 1315-1376) in Masechet Pesahim, and from the Gemara’s discussion in Masechet Baba Batra (93), that even wine which is not intoxicating is suitable for the four cups of wine at the Seder. He applies this conclusion to grape juice, as well.
As a practical matter, Hacham Ovadia, as well as Hacham Bension Abba Shaul (Israel, 1923-1998), ruled that one should preferably use wine for the four cups, even if this poses some difficulty, but women, and those for whom drinking wine is very difficult, may use grape juice. Hacham Bension adds that a person who cannot drink four cups of wine should use wine for at least the final cup, as he will go to sleep soon afterward. Another option is to mix wine and grape juice for the four cups.
It should be noted that Rav Moshe Feinstein (Russia-New York, 1895-1986) is cited as having ruled against the use of grape juice for the four cups at the Seder. However, as mentioned, the leading Sephardic Sages of the last generation – Hacham Ovadia Yosef and Hachan Bension – allowed the use of grape juice for women and for those who have considerable difficulty drinking four cups of wine.
Summary: One should use wine, and not grape juice, for the four cups at the Seder, but women, as well as men for whom drinking four cups of wine poses considerable difficulty, may use grape juice. Preferably, they should mix wine and grape juice, or drink wine for at least the fourth of the four cups.
