Don't know if this is new bug in beta 4 since I didn't play with previous ones too much.
I have 3 vertical videos next to each other, in standard FullHD 16:9 frame.
If they are not touching each other, everything is ok.
If the video that is on top layer covers the one below even by only 1 pixel, suddenly the bottom video isn't displayed at all - it's just black!
If I decrease the opacity of top vid from 100% to 99%, then suddenly the bottom video is displayed correctly again! Changing blending from normal to something else works too (but this of course isn't a fix).
It's not just display bug - the video renders this way (with empty black space instead of bottom vido) too!
Since I did some coding in my time
I bet you have a function somewhere that checks if the video is covered to speed up editing/rendering if it's not visible. The only problem is the function doesn't check properly if the whole video is covered, or more likely the returned value isn't handled properly...