The only significant practical difference between communal customs regarding the laws of the Seder pertains to the berakha over the second and fourth cups.
Many Rishonim maintain that “borei pri ha-gefen” must be recited over each of the four cups, even though our attention is not diverted from one cup to the next, because each cup is a mitzva in its own right. This is the opinion of R. Natronai Gaon, R. Amram Gaon, Rif, Rambam, Maharitz (a leading Yemeni authority), and Rema, and it is the practice of Ashkenazim and of Yemeni Jews who follow Rambam.
However, Rosh maintains that “ha-gefen” must only be recited before the first and third cups. The berakha over the first cup covers the second cup because there is nothing between them to divert our attention. We recite a berakha over the third cup because it follows Birkat Ha-mazon and a berakha is always recited over wine we drink after Birkat Ha-mazon, even if “ha-gefen” was recited earlier in the meal, because Birkat Ha-mazon serves as a berakha aĥarona for the wine one drinks during the meal. The berakha over the third cup covers the fourth cup as well. R. Yona and Rashba also maintain that “ha-gefen” is recited over the first and third cups only. SA rules accordingly, and this is the Sephardic custom.
There are also differing opinions among Rishonim regarding the berakha aĥarona over the wine. In practice, however, there is a consensus not to recite a berakha aĥarona after each cup of wine. Rather, Birkat Ha-mazon covers the first two cups, and the berakha aĥarona (“al ha-gefen”) recited after the fourth cup covers both the third and fourth cups.
. It is worth adding here that many families customarily prolong the recitation of the Hagada beyond seventy-two minutes, which is the time it takes to digest and is generally considered the amount of time that constitutes an interruption, after which one may not recite a berakha aĥarona. It would seem, then, that an extended recitation of the Hagada would mean that Birkat Ha-mazon does not cover the first cup of wine, in which case one will not have recited a berakha aĥarona over the first cup of wine. Moreover, according to MA 184:9, if one waited for this amount of time between the first and second cups, he must recite a new berakha, which contradicts the ruling of SA that one need not recite a new berakha over the second cup (since the berakha on the first cup covers it). Additionally, how does this issue fit with Rashbam’s ruling that the berakha recited on the karpas also covers the maror? How can this work if more than seventy-two minutes have elapsed? This question is also raised in SA 473:6, which takes the view of Rashbam into consideration. Furthermore, the first cup is also kiddush, which must be part of a meal. When there is such a long break, perhaps kiddush must be recited again, so that it is part of the meal. Because of all of these problems, some poskim maintain that one should be careful to recite the Hagada in less than seventy-two minutes (see Sidur Pesaĥ Ke-hilkhato 3:9). Notwithstanding these concerns, many people still prolong the recitation of the Hagada for more than seventy-two minutes (see Mikra’ei Kodesh 2:30, Hilkhot Ĥag Be-ĥag 20:5, and Responsa Ĥazon Ovadia §11, which justify this practice based on various rationales, such as explaining that the berakha aĥarona on the fourth cup also covers the first cup). In my humble opinion, the entire issue of a significant interruption (“hefsek”) in a meal only applies when one engages in unrelated activities (even if he does not take his mind off the meal). In this case, though, he is not dealing with unrelated issues; rather, he is involved in the Seder and the Hagada. Therefore, since the second cup is poured right at the beginning of the recitation of the Hagada, and since the entire recitation of the Hagada is connected to the second cup, the time between the first and second cups is not considered a hefsek. Consequently, there is no hefsek between kiddush and the meal; the recitation of the Hagada is considered part of the Seder, since telling the story of the Exodus is an integral part of the mitzva of eating matza. Similarly, telling the Exodus story is also inherently connected to the mitzva of eating maror, so the recitation of the Hagada does not constitute a hefsek between the eating of karpas and the eating of maror (and the berakha on karpas covers the maror as well).