WordPress 4.0 comes with updated MediaElement.js 2.15.0, but, as it turns out this version has several serious bugs related to videos resizing. Soon after 2.15.1 was released to fix those issues, but for some reason WordPress 4.0 stick to broken version. 2.15.1 was released one month before WP 4.0 was.
Until updated WordPress is released, this poses a big problem to any plugin relying on this library, and I have discovered the problem through my Smart Video Playlists plugin. To remedy the problem, I had to make a very important change not to rely on WordPress built in MediaLibrary. So, from now on, my Smart Audio Playlists and Smart Video Playlists plugins will have own, and always latest versions of MediaLibrary.js to prevent further problems with WordPress built in version.
Smart Video Playlists 1.1 and Smart Audio Playlists 2.1 are released to fix the issues. Both are tested with WordPress 3.9.2 and 4.0. If you have any more issues, please report them in the forums.