If one forgot and did not eat the Afikomen until he had already washed his hands prior to the Birkat haMazon, or until he said the Zimun (Rabbotai Nivarech), or even if he had already recited the Birkat HaMazon but prior to the third cup of wine, the Avi Ezri and the Rosh write that he should wash his hands again, and recite the motzi and eat it. If, however, he already said the blessing over the third cup of wine, he should not eat it since this necessitates that he recite reciting the Birkat HaMazon and then drinking another cup of wine, and it would now appear that he is adding to the four cups of wine. It has also been established that one should not drink between the third and fourth cups of wine. Also, since all our matzah is shemurot, one can consider the last bit of matzah that he ate as the Afikomen. Rabbi Peretz wrote that if he hasn’t recited the Zimun yet, he can still eat the Afikomen without a blessing even though he has already ended the meal. In this situation it is different because the Afikomen is a commandment that God has placed upon him and we depend upon this. After he drinks the cup of wine over which he recited the Birkat HaMazon, he pours the fourth cup and completes Hallel. In Midrash Tehilim, it is written that there should be at least three people reciting psalms so that one can recite and two can respond, hodo l'adonai ki tov. The Rosh writes that if it is impossible for him to have three people present, he his wife and his minor son is sufficient!