4-8 cups of coffee is excessive and potentially unhealthy in my opinion - just because it doesn't have adverse effects on one person, it doesn't mean that its not a bad idea. But 0.5-1 cups in the evening sounds quite reasonable, so if it helps you, go for it. I personally have found that coffee and piano are mutually exclusive... I once drank coffee right before a lesson, and I found myself jumbling the fingerwork in a fast piece and unable to maintain concentration in a slow piece (not that I normally play perfectly, but this was unprecedented), and finally I actually burst out laughing while I was playing (to my teacher's horror) when I realized that the caffeine was the problem!