WP 3.9 TinyMCE no longer loads on category description editor

I was having the same issue and the problem was actually coming from the way the js was dynamically generating tinyMCE.

Prior to v.4, it was:

tinymce.EditorManager.execCommand('mceAddControl', true, id);

With 4, you need to use:

tinymce.EditorManager.execCommand('mceAddEditor', true, id);

Have a look at wherever your function ‘wp_tiny_mce’ is – might be in there.