Both men and women are obligated in the Misva of "Se’uda Rebi’it" ("the fourth meal"), the meal eaten on Mosa’eh Shabbat. When is the final time by which one must eat this meal after Shabbat?
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Vayeseh (listen to audio recording for precise citation), writes that one should preferably conduct the Se’uda Rebi’it within four hours after the end of Shabbat. ("Hour" refers to a Halachic hour, which changes depending on the season.) In the winter months, for example, when Shabbat ends at approximately 5:00-5:30 PM, one would be required to eat the Se’uda Rebi’it by sometime between 8:00 and 8:30. According to Rav Haim Palachi (Turkey, 1788-1869), in his Kaf Ha’haim, one does not fulfill the Misva of Se’uda Rebi’it if he eats the meal after four hours have passed since the conclusion of Shabbat. The Ben Ish Hai, however, maintained that the deadline of four hours applies only on the level of "Misva Min Ha’mubhar" (the highest standard of Misva performance). One who did not eat the meal before the end of four hours may, according to the Ben Ish Hai, do so until Hasot (midnight as defined by Halacha).
Yet a third position was taken by the Gaon of Vilna (Rabbi Eliyahu of Vilna, 1720-1797), who held that one may eat the Se’uda Rebi’it throughout the night, until daybreak. It is told that the Vilna Gaon once took ill on Mosa’eh Shabbat, and was unable to eat the Se’uda Rebi’it. He went to sleep, and later in the night he woke up and felt capable of eating. The rabbi asked his family to look outside and see if the sun’s rays were visible, and they saw that no sunlight had yet appeared on the eastern horizon. The Gaon then partook of a Kezayit of bread to fulfill the obligation of Se’uda Rebi’it, clearly demonstrating that in his view, one who did not eat the meal before Hasot may still do so, until daybreak. This is the view followed by Hacham Ovadia Yosef, both in his Yehave Da’at (vol. 4) and in Halichot Olam (listen to audio for precise citation).
Interestingly, there is another debate between the Ben Ish Hai and Hacham Ovadia concerning the recitation of the verse, "Magdil Yeshu’ot Malko" in Birkat Hamazon after the Se’uda Rebi’it. According to the Ben Ish Hai, one should recite the text of "Migdol Yeshu’ot Malko," just as we do on Shabbat, Rosh Hodesh and festivals. Hacham Ovadia, however, held that after the Se’uda Rebi’it one should recite "Magdil," just as we do on ordinary weekdays. Each person should follow his family custom in this regard.
Summary: Both men and women are obligated to eat the Se’uda Rebi’it meal on Mosa’eh Shabbat. Ideally, one should eat this meal within four hours after the conclusion of Shabbat, or at least before Hasot. Nevertheless, one who did not eat the Se’uda Rebi’it before Hasot may do so even later, until daybreak. Different opinions exist as to whether one should recite "Magdil Yeshu’ot Malko" or "Migdol Yeshu’ot Malko" in Birkat Hamazon after the Se’uda Rebi’it; each person should follow his family custom.
