Ok, the trick is to open file explorer, locate the file, do a right click and pick the application to Open With. Mp4 like mkv are like video containers. Within those you have tracks codified by what is called Codecs. So what's happening to your mp4 is that probably the video codec used is not installed in your Surface 2. That's the case with MKV files. They are not supported by MS by default. At this moment the only thing you can do is to use Apps that contain their own Codecs configured and "installed", like PowerDVD, Multimedia8, etc. The link posted by you points to the x86 version of the extra Codecs package needed by Windows to deal with some formats like DivX and Xvid. My guess is that your mp4 was created using one of these two. :wink:
All this counting with your mp4 file not being copy protected.