Be careful about updating if you use third party Plug-ins
Contact the supplier first before you update to 10.0.4
Magic Bullet have tweeted the following:
“The FCPX 10.0.4 update breaks the preview functionality for Magic Bullet Looks & other plugins. Do NOT update. We’re looking into it.”
Make sure you check with your Plug-in suppliers first.
My suggestion: If you have not updated yet – hold off for awhile