sorry epond- I am unsure how to answer your questions
.
khisanthax- well I don't think you could double up... 4 cups, to my knowledge, won't dissolve into what would be close to 1L of water. If you double the sugar than the yeast will likely die out before the sugar is used up, and if you double the yeast you will get more bubbles but for a shorter life span.
Having said that, I think if you used a bigger container you could double everything... but I don't think it would extend the life, only the CO2 production.