Normally if you have wine and refill your glass during the meal you don't make a new borei p'ri hagafen. Why do we (at least Ashkenazim) say a new b'racha for each cup at the seder? I think I've seen an explanation somewhere about them having four different purposes, but I couldn't find it in any of the haggadot I have handy (and I thought that's where I read it).
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The Magen Avraham (OC 474:1 and similarly in Taz there) explains that each of the four cups is a separate mitzva, and it is therefore considered as if he had in mind specifically not to exempt the other cups with his Brocha. The Pri Megadim adds that it is preferable to actually have this in explicitly in mind.
The Minchas Shlomo (vol. 1 18:6) questions this explanation, for granted that each cup is a separate mitzva but the question was why we make a separate "hagofen" for each cup. "Hagofen" is not a brocha recited before doing a mitzva (bircas hamitzvos) , but rather a brocha said before eating (bircas hanenen) and so what relevance is there to the fact that each cup is its own mitzva? He explains that because each cup is an independent mitzva, the Sages established that one should not have the subsequent cups in mind while reciting the blessing in order to give each cup it's own importance. See there are length.
The Gra there (quoted by Mishna Berura :4) brings an alternative explanation, that since one may not drink between the first and second cup (after he starts reciting the hagada) this is considered an interruption that requires a new Brocha.