Why is there not (or maybe there is) a requirement to rinse the cup used for the arba kosos, before each individual refilling. Is each one of the arba kosos not a separate kos shel bracha that requires hadacha and shetifa. Do we rely on the initial one before kiddush for all four? If so, why/

Shalom, Thank you for your question. The issue of whether one need to wash and rinse the cup before each of the four cups has been widely discussed by the Poskim (see Kaf HaChaim 473,1 for many sources). There are three different practices. Some people do not rinse out the cups at all (the Mishna Brurah 473, 68 writes that because the cup was washed before kiddush it does not need to be washed again). Some rinse and wash the cups for kiddush, and then for the third cup (of birkat ha’mazon (grace after meals)). This is because those two cups are certainly “cups of blessings” (and in fact the Sephardi custom is to say the blessing of “ha’gefen” only over those two cups). Others, based on the kabbalah, wash the cup before filling it for all the four times during the seder. They often have a bowl and water ready at the Seder table for this purpose. You should follow your family custom. Chag Semach.