Most of the discussion revolving around the four cups of wine we drink during the seder, tends to focus on what is the nature of this chiyuv, is it a din in cheirus, are women obligated, should one lean etc. However, there are a couple of interesting shailahs which come up that are relevent to everyday halacha, specifically in the realm of hilchos berachos.
Two shailahs that come to mind are the following. 1) why do we need to make a new beracha after each cup-why don't we say that just like the ha'adama of karpas works for marror, so too the hagefen of the first cup works for all cups. 2) what is considered to be the beracha acharona of the 4 cups. Is bentching the beracha acharona for cups 1&2 and the "al hagefen" after nirtzah works on cups 3&4 or would we say that "al hagefen" is the beracha achrona on all 4 cups.
Although these are interesting questions, I am not going to focus on them. I just mention then as an aside. The question I want to focus on is why don't we need to say "al hagefen" after the first cup. We pasken that if too much time has passed between eating and the beracha achrona then a beracha achrona can't be said. The amount of time is called a "shiur ikkul"-the time it takes for food to digest. We generally assume it is 72 minutes but the poskim point out that for a drink or quick snack the time can be shorter. I looked briefly in the Piskei Teshuvos and he brings down opinions that for drinking the time limit can be 30 minutes or as little as 11 minutes. However, even if one is meikil that you have 72 minutes, most of the time from the time you finish karpas until you drink the 2nd cup, more than 72 minutes have passed. If so, why don't we say al hagefen after cup#1?
There are a number of answers mentioned in the poskim.
1) Aishel Avraham M'Butchatch
The Aishel Avraham (Butchatcher) in 474:1 writes that where as the requirement for a new beracha rishina is based on hesach hada'as, beracha acharona is based on different criteria. If you have decided that the meal is over then you need a beracha acharona. However, if in your mind taht meal is still going on, even if a long period of time has elapsed, you don't need to say a beracha acharona. Therefore, at the seder everyone knows that there are more cups to be drunk and noone thinks they are finished for the night. Consequently, no beracha acharona is said.
The kasha I have on this is that thsi not pashtus in Shulchan Aruch. The Shulchan Aruch says it is based on shiur ikkul not whether you finished your meal.
Shulchan Aruch HaRav
The Tzitz Eliezer (Chelek 12 siman 1 ) brings the Shulchan Aruch HaRav (siman 474) who says we don't make a beracha achrona on the first cup beacuse as long as the "stomach is open, the food doesn't digest". I understand this to mean that as long as you are still involved in eating (even if you are not actually eating) the food is not considered digested. I don't know if this is the same sevarah as the Eishel Avraham but it is close to the same idea.
Rav Shlomo Zalman Aurbach zt"l
Rav Shlomo Zalman discusses this in Mincha Shlomo Siman 18:11. It is also brought in the Halichos Shlomo on Pesach and in the haggada they put out with his chiddushim. He first bringe the mishna berura that it is assur to put yourself into a situation where you will lose out the beracha acharona. He says a possible answer is that how do we have kiddush b'mokom seudah during the seder. There is a hefsek between kiddush and the meal. He answers that the whole seder is one hemshech and is all connected to kiddush. Howeevr, he writes that if you would make an al hagefen you would break this hemshech and lose your kiddush b'mokom seudah.
Interestingly, it appears he wasn't convinced of this teretz because in his haggada it is brought down that he was makpid to eat less than a k'zayis of karpas during maggid. This way 72 minutes did not pass without eating. However, he did not tell others to do this and it appears it was his personal hanhaga. Furthermore, his son is quoted as saying that in his later years when he could not eat more karpas he would actually say an al hagefen.
The other interesting thing I picked up from here is that eating food works for liquids.
Rav Shternbach (chelek 1 siman 305)
Someone wanted to suggest that we drink less than a reviis of wine for the first cup. Rav Shternbach was very against this and quite adamant about not changing anything. He suggested that the reason there is no need to say al hagefen is because the shiur ikkul was only said for when you drink l'ha'na'ah. Herre you are drinking l'mitzvah and it is toloi on hesech hada'as. Also, if you say al hagefen it is hefsek from kiddush. The main thing is not to be motzi la'az on ourf minhag and this is our mesorah of what to do and since it is not mentioned anywhere to say "al hagefen we can't come up with kuntzim to change it.
I saw in Rav Elyshiv's haggada that he says that if you know that more than an hour will pass betwene karpas and cup #2 , then you drink some water or say al hagefen.
This is not like Rav Shlomo Zalman in 2 ways. 1) Rav Shlomo Zalman didn't pasken this way for others 2) Rav Shlomo Zalman ate karpas. Rav Elyashiv says drink water and don't know what he would say about eating karpas.