Well just drop Drupal and Kb and you're good to go, there's an integration plugin for phpBB+WordPress that does exactly what you need. It may be a good place to start, because the concept will remain the same in each system - check if there's a login present from one of the others, and if so add the same login to the currently active system.

That's an awful lot of applications to be running on a single site under a single login though - maybe before you go through the headache of writing an extension for each, you should examine if you really need one site running four separate entities like that.