We always go at park opening. We hit all the best slides before the crowd builds and are usually ready to leave by early afternoon. I know that many people say that if you go when we usually leave that the crowd is thinning out, but I can tell you that when we leave, the crowds are usually huge, and we're glad to have been there early. I've never waited til late in the afternoon (after 4), but that seems like a possibility, too.
Julie