Your screenshot looks fine, but it does not tell us much about what you expect and what you received from export. For example, if part of the image is missing, show a frame of the full image from the 4K video and a frame from the same time of the 1080p video.
Shotcut does not display video full screen unless you use external monitoring. Use a media player if that is what you are looking for.
Sorry for bothering and thanks to all for help. Now I know where the problem is. I use k-lite codepack with windows media player classic and I had it on half size and that was the issue. After changing it to stretch to window it is OK.
I find using VLC for displaying and/or debugging much better that Microsoft’s own apps. These sometimes ask for codecs to be purchased, whereas VLC is quite happy with almost everything I throw at it.