Hi all I experience the same problem with my 930 on Denim.
Most videos that I record I can't play on my computer, I get an error message that says it can't read that file. They're all MP4 files.
I can play the videos with VLC though but it's not useful when I want to edit them on Moviemaker.
As far as 4 K even with VLC they are choppy, not a smooth play.
I found that if I trim them with Lumia video trimmer the computer sees them.
I don't want to have trim each video to use them.
I kind of figured it might have something to do with the settings.
So if anybody has figured out how to fix this I would greatly appreciate it.
Thank you