When I open my 4K file, it "looks" like it has physical black bars.
When I used to convert files to WMV, Expression encoder had a way to change the size of the height without stretching or anything. So a 1920 x 1080 file would end up being 1920 x 800 without the bars. Now on my screen it did not stretch to full screen. There were still the bars on screen but from what I can tell they weren't in the file then. Would think you wouldn't want the back bars IN the file? Any way to strip them out?