Yiannis1970:
1. Playing 1366 x 768 or 1920 x 1080 or any other resolution 16:9 (16:10), you do exactly the opposite of what you are after. The ding line is more narrow not wider. If you want a wider ding line the only resolution to play is 1024 x 768 or custom resolutions as Scott mentioned before.
2. Of course you can watch 4K movies on a 1920 x 1080 monitor. The content will be downscaled to monitor's native resolution. If you use an external player like a Blu Ray and feed the monitor, check that your HDMI supports 4K resolutions at 60 Hz (i don't say display port cause usually BD do not provide one). The HDR is a bitter bitter story which is better leaving out of the frame.
So don't believe my eyes, only what you tell me? Playing 1366 gives me a ding line 3x that of 1920. I see that because I can ding early, ding perfect, or ding late. When I ding early, I can still see ding line to the right the meter line. Perfect ding obscures the ding line, late ding leaves part of ding line to the left of the meter line. At 1920 it's ding or nothing.
Your point of 1024 providing the best ding line is correct of course, but I prefer to enjoy full screen.
If you're downscaling, you're not watching native 4K video. Symantecs.