I've been working on implementing the latest CKEditor version into vBulletin 4 and need some testers in about 2-3 weeks to test functionality. If you want to help testing then please post in this thread and I'll send you a private message whenever a test version is ready.
I have to implement a couple more things: custom BBCode functionality, server interaction when switching between source/WYSIWYG and vBulletin's plugins (such as smileys, attachment, etc.)
Most of it will be a matter of porting over the existing code to work with CKEditor 4.