All Halachic authorities agree that one recites the Beracha of "Boreh Peri Ha’gefen" over the first and third of the four cups of wine at the Seder. Over the first cup of wine, of course, a Beracha is required, because one has not drunk any wine before this cup, and a Beracha is obviously required before drinking. Likewise, the third cup of wine is drunk after Birkat Ha’mazon, and the recitation of Birkat Ha’mazon effectively ends the "coverage" of all Berachot recited previously, such that new Berachot are recited before one eats or drinks after Birkat Ha’mazon.
The more complicated issue is the recitation of this Beracha before drinking the other two cups of wine. Seemingly, the Beracha recited over the first cup should cover the second cup, and the Beracha recited over the third cup should cover the fourth cup. At first glance, then, no Beracha should be required over the second and fourth cups.
Nevertheless, the Rif (Rav Yishak of Fez, Morocco, 1013-1103) rules that one must recite a Beracha of "Boreh Peri Ha’gefen" on each of the four cups. He bases this ruling on the Gemara’s comment in Masechet Pesahim that each of the four cups constitutes a separate Misva. If each cup is its own Misva, the Rif reasons, then each cup should require its own Beracha.
The Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327), however, questions this rationale, noting that we recite only one Beracha when putting on Tefillin ("Le’haniah Tefillin") even though the Tefillin Shel Rosh and Tefillin Shel Yad constitute two separate Misvot. The fact that the four cups constitute independent Misvot should not have any effect on the recitation of Berachot.
The Rif also draws proof from the Gemara in Masechet Hulin, which implies that one must recite a new Beracha when drinking after Birkat Ha’mazon. Since one cannot recite a Beracha while reciting Birkat Ha’mazon – as this would constitute a Hefsek (improper interruption) in the middle of the Birkat Ha’mazon – he must recite a new Beracha when he drinks afterward. Likewise, the Rif comments, since one cannot interrupt the reading of the Haggada to recite a Beracha, he must recite a new Beracha of "Gefen" when he drinks the second and fourth cups.
The Rosh questions this argument, too, claiming that there is no comparison in this regard between the reading of the Haggada and Birkat Ha’mazon. The Gemara states that if one interrupts a meal to pray Minha, when he returns he resumes his meal without having to recite new Berachot. Likewise, if one interrupts his drinking to read the Haggada, he does not have to recite a new Beracha when he then drinks again. Birkat Ha’mazon is unique, the Rosh argues, because when one decides to recite Birkat Ha’mazon, he establishes that he no longer plans on eating. Therefore, if he then decides to eat or drink afterward, he must recite a new Beracha. This quite obviously does not apply in the case of reading the Haggada.
In practice, Ashkenazim follow the ruling of the Rif, which is also the position taken by Tosafot, and they recite a Beracha on each of the four cups of wine at the Seder. Despite the Rosh’s compelling questions on the Rif’s rationale, this is the practice of the Ashkenazim. The Taz (Rav David Halevi Segal, Poland, 1586-1667) explains this view by suggesting that perhaps there is an implicit intention when reciting the Beracha over each cup that the Beracha should cover only that cup. The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), however, dismisses this approach.
In any event, Sepharadim follow the ruling of Maran, in the Shulhan Aruch (Orah Haim 474:1), where he accepts the Rosh’s view, that no Beracha is recited over the second and fourth cups. The second cup is covered by the Beracha recited over the first cup, and the fourth cup is covered by the Beracha recited over the third cup, and no separate Beracha is required.
Summary: According to accepted Sephardic practice, the Beracha of "Boreh Peri Ha’gefen" is recited over the first and third of the four cups at the Seder, but not on the second and fourth cups.