I just ran a quick check on the db and I have 120 accounts with usernames that have 5 or less characters. As far as 'good' usernames go, that's a little more subjective but a large number of them are good in my book. By this I mean that they aren't some templated thing like +<4 numbers>, all these usernames were scraped from other places on the web and then registered on reddit. Naturally some that were scraped arent good but there are a large number that make sense and plenty that have made me laugh when I first saw them.