If a person ate a Ke’zayit or more of bread to the point of "Sebi’a" (satiation), meaning, he ate a quantity that gave him a feeling of satiation, he must recite Birkat Ha’mazon by force of Torah obligation. In a case of somebody who cannot remember whether or not he recited Birkat Ha’mazon, we apply the rule of "Safek De’Orayta Le’humra," which means that we must follow the stringent possibility in situations of doubt involving a Torah obligation. Since this individual is potentially required by force of Torah obligation to recite Birkat Ha’mazon, he must assume he had not recited it, and recite it now.
The question arises as to whether this applies as well to the fourth Beracha of Birkat Ha’mazon, the Beracha of "Ha’tob Ve’ha’metib." As opposed to the first three Berachot of Birkat Ha’mazon, which one is required to recite by force of Torah obligation, the fourth Beracha was instituted by the Sages of Yabneh. The recitation of this Beracha constitutes a Rabbinic obligation, and we should thus, at first glance, apply the rule of "Safek De’rabbanan Le’kula," which means that we follow the lenient possibility in situations of uncertainty involving a Rabbinic obligation. According to this rationale, a person who cannot remember whether he recited Birkat Ha’mazon must recite the first three Berachot, but not the fourth Beracha.
Some authorities, however, including the Keneset Ha’gedola and the Olat Tamid, rule that despite the Rabbinic origin of this fourth Beracha, it must be recited even in situations of uncertainty. According to this view, we must be concerned that if people are instructed under certain circumstances to recite only the first three Berachot of Birkat Ha’mazon, they may consider the fourth Beracha unimportant. They will approach this Beracha without the seriousness it deserves, to the point where they might even omit it altogether. Therefore, according to this position, a person who does not remember whether he recited Birkat Ha’mazon must recite all four Berachot of Birkat Ha’mazon.
Many other authorities, including the Kaf Ha’haim, the Ben Ish Hai and the Peri Hadash, disagree. They argue that since this situation arises only on rare occasions, we need not be concerned that people will belittle the importance of the fourth Beracha of Birkat Ha’mazon. It is only on infrequent occasions that a person forgets whether or not he recited Birkat Ha’mazon, and therefore reciting only the first three Berachot on these occasions will not undermine the seriousness with which he generally approaches the fourth Beracha.
This is, indeed, the accepted Halacha, and thus one who cannot remember whether or not he recited Birkat Ha’mazon should recite only the first three Berachot of Birkat Ha’mazon.
Summary: If a person ate a quantity of bread that requires him to recite Birkat Ha’mazon, but he cannot remember whether or not he already recited it, he must recite the first three Berachot, but not the Beracha of "Ha’tob Ve’ha’metib."
