I'm going to fix this for you and suggest the 4's are a bit better place for learning. Cat5's are really just about surviving and getting a taste of bike racing to see if you want to do it, but you'll have so many different levels of people, ranging from the commuter cyclist showing up with a t-shirt and tennis shoes, to the sandbagger who just wants to be king of the cat5s, that it's not really bike racing and I don't know how much you'll learn. Cat4's have their own problems too (less chess, more chase), but most of the people there want to race and are a little more serious.

Actually, although I can't speak for the 2's and 1's, you'll probably learn something in just about every category...