The hagadda has us say the Mein Shalosh after-bracha for wine for the 4th cup. Why do we say the after-bracha for that cup and not for each of the other 3 cups? Why are we not concerned about losing the opportunity to say the after-bracha over the first cup, for example, since the meal does not begin until the bread and it is not one long bout of drinking but rather two cups separated by considerable time.
See Shulchan Aruch and the mishna Berurah 473:2 and 474 (M"B 3-5).
The first kos gets a bracha because it constitutes kiddush hayom and is considered part of the seudah, so it doesn't get an after-bracha. The second kos doesn't get any bracha at all since it's between the first and the seudah itself and is covered by the brachos from those two.
The third would normally get a bracha acharonah right after birkat hamazon, but because we already have in mind that there's a fourth kos coming, we delay making it because there's no concept of hefsek/hesech hada'as regarding the bracha achronah (d'ain hesech hada'as kovea bracha le'acharav).
The fourth kos has it's bracha rishonah covered by the third.