It has come to our attention that there is a bug in 4.0.4 that allows guests to see threads posted in private forums when using the Recent Threads Widget. Please see this bug report for the fix:
http://tracker.vbulletin.com/browse/VBIV-7562
If you do not have time to patch that file, then we suggest that you disable the Recent Threads widget until a patch is available. We apologize for the inconvenience but we wanted to let you know about this as soon as possible.
More...