In my WP4.0 installation somehow I wasn’t able to drag & drop widgets anymore.
What did I do to solve this problem? Copied all WP4.0 files over to the folder I had WP installed.
In other words, I unzipped the downloaded zip file of WP4.0 into the folder I had my WP project installed.
Said “Yes” to Replace All, refreshed the Widgets page and voilá, they worked again and I was able to drag & drop as usual.