I have installed the 7.x-2.x-dev version of the WYSIWYG module and the CKEditor v4.0 library. After install, all is fine and the CKEditor library is detected.
After configuring which buttons should appear in the WYSIWYG CKEditor profile, I end up with an empty toolbar. No errors are given in Firebug console.
I have also tried using http://drupal.org/project/wysiwyg_ckeditor module with the same result. (obviously, this was installed "alongside WYSIWYG module".
Any ideas or alternatives I could try in order to use the latest CKEditor? It is the "FULL" download package I used not a customised one.