I’m also running WP 3.9.1 and ACF 4.3.8 and am having mixed results with the WYSIWYG editor. On one page that I have it set it doesn’t work, but on all the others it does.
The main difference between the various pages is that the one where the WYSIWYG editor doesn’t display has an ACF Google Map field included. If I remove the Google Map field and reload the page editor, the WYSIWYG editor then displays properly.
When I inspect the page, I get the following error:
TypeError: google.load is not a function
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