Hey there,
This is a common issue for most people when they download Ubuntu 18.04. The problem is (most of the times) Ubuntu's default app doesn't have the proper codecs or decoders. So basically they get a video file format it goes to open and it doesn't have a single clue what to do with it. Hence the extremely blurry 4K pixels. Hence why VLC works is because they have the necessary codecs and decoders to translate the amazing 4K pixel blocks into an actual video. The work around for this is simply installing the necessary files. Open the terminal and copy and paste the two lines down to download the necessary codec and decoders to watch the MPEG-4 videos.
Code:
sudo apt install libdvdnav4 libdvdread4 gstreamer1.0-plugins-bad gstreamer1.0-plugins-ugly libdvd-pkg
sudo apt install ubuntu-restricted-extras
hope this helps!
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