Actually, Java at one time did have a fifth level of visibility called private protected.

It was different, but it wasn't a good idea, for the same reason that I think edger's paranoid isn't a good idea:

It's unnecessary. You are writing that class, so if you don't want that method or class member to be used by other instances of the class, then don't! Not doing something stupid is what programming is all about. Defensive programming is a good thing, but using another instance's methods or members is a conscious decision you make, not a thinko (if it isn't, switch your brain on while coding).

Java's visibility levels are not really a security feature, they're mainly there to tell other programmers which parts of a class are for external usage, and which parts aren't (and therefore shouldn't be touched unless you know what you're doing). If someone else works intensively on the same parts of source as you, you're having an organizational problem, I'd say, but if it's really necessary, you can still leave a comment to tell them what not do do.