Development cannot be correct. I experienced the same problem with the Add Media button suddenly not working. Also the Link button is not working. Tried it on different pages with same result. Re-installed WP 4.0 and have same situation. Although I can create links manually, I’m not smart enough to know a workaround for getting an image from the Library onto my page without the Add Media button to enable automatically entering the address of the file. Reported to trac as #30060.
