I use wordpress a lot, i develop for it a lot too!
I have a client website that i have added 10 custom meta boxes to the post edit screen in functions.php . All the meta boxes have an editor using wp_editor()
This all worked find in wordpress 3.8 and 3.9 but now the site has been upgraded to wordpress 4 the editor is refusing to show the visual/text tabs on the editor pane.
I have verified this by deactivating all plugins on my development copy, and the issue persisted. I then installed a second copy of wordpress but 3.9 and the tabs came back!
Any idea why this is happening on wordpress 4?