According to the Rema (Shulchan Aruch, Even Haezer 62:9), one uses two cups of wine at a sheva berachos. However, if a sheva berachos were to be held on seder night, there would be a problem, since one may not add to the number of cups of wine drunk after eating the afikoman. Rav Yaakov Emden discusses this problem in his siddur (Beis Yaakov, seder hagadah, netiv 10, paragraph 25) and he states several possible solutions:  1) the participants at the seder recite birchas hamazon over their own cups of wine and the sheva berachos are recited on the birchas hamazon cup of the choson. 2) to follow the opinion of the Shulchan Aruch, which states that one always uses the same cup of wine for both birchas hamazon and the sheva berachos. 3) to use the cup of wine for birchas hamazon as the third cup and the cup of wine for the sheva berachos as the fourth cup. (see also Sha’arim Metzuyanim Bahalachah on the Kitzur Shulchan Aruch 119:15)