You too can be a Cat 4!! To move from Cat 5 (buying your $60 annual license is the only requirement, don't even have to join a club) to Cat 4 all you need to do is start 10 qualifying mass start races. You don't even have to finish. (Of course you won't be having any fun as a 4 if this is you)
To move up from there you need to start winning or placing in the top 10 in the Cat4 races, and so on.