Bringing this thread back to life.
I think it might be easier to change the User table columns in WP to match with phpBB and possiblity 4images. From what I can tell, they both use session variables to determine who is logged in. However, they determine the user access level based off of the base user information session variable. So, the same amount of variables might need to be passed in order for the login to work.
Check out the auth.php and session.php files in phpBB includes folder.
I think you would have to change the register screen in WP to call the register action form in phpBB, so it would add the user to both places. The only changes that should pass over from one area would be the profile info and password. Changes to the preferences would only happen when in the particular application.
Once the tables are sync'd then you would have to change each applications Log-in areas to create a session variable for each application or change the session variable names so that they are all the same and can be used by all applications. Not sure if the later would work with password part, if they use different encryption methods.
From what I see, this would be more than a my-hacks addition.
I think it would be nice if part of the setup options if you could designate your blog, gallery and forum programs, so that the login's could be combined.