You mention VLC, it has the ability to export frames built in to it, just search for VLC export frame and you can find plenty of tutorials for doing that. From the way you have written, it sounds like you are trying to view a 4k video on a 1080p monitor, which is why your screen grab may be coming out as 1080 x 1920.
This thread may help! it lists some other options: https://www.dpreview.com/forums/thread/4054568