//EDIT: Now that I got your file I see what happened there. The image sequence strip is scaled to 50%, if you select the strip and go into the sidepanel of the Sequence Editor, under Transform you can see Scale X and Y are set to
0.500. A strip is not necessarily always filling the full output size, for example if you want to overlay a smaller image over a background sequence and that's why you can scale it different from the output size.
This most likely happened to your sequence because Blender's output resolution is set to 1920×1080 by default. When you added the image sequence then, it was scaled down to fit the render size. When you changed the output resolution to 3840×2160 afterwards, the image sequence kept the scale of 50%. This was not always the case, in older versions the strip automatically adjusted to the new output size. The annoying thing was that when the strip had a ratio of 16:9 and you changed the output to something like 1:1 or 3:2, the clip was stretched and changed its original ratio, which was not always desired as default behavior.
Old answer, maybe someone else can find useful information in here:
The black area is simply the window space outside of the image size. If your image strip was smaller than the complete render size, it would either show most obviously a transparent area where it is not filling the space (as long as there is no image strip below which would then be visible) when Color & Alpha is enabled in the preview or you would at least see a thin dashed line where the actual image border is if only Color was enabled (as it is in your screenshot, but maybe the dashed line is not visible because it is out of the window's bounds):
Since you only show the Preview image in the sequencer and not how it looks when you render an image by hitting F12 and if there are still black borders (which is what is shown in the question you are linking to, the rendered output not the preview), I would suppose this is the solution to your black border problem.
About the quality of your image sequence I cannot say anything without examining the files myself, however if you probably have set the percentage in the Output Properties to something lower than 100%, perhaps because you want to render your 3840×2160 images with only 50% or 25%, then the preview image will not change the size it is displayed, but show a zoomed in image in the same size but with a lower resolution.
In the following examples I've put a 1920×1080 movie strip into the sequencer which is previewed in 1:2. The Output Properties are set to render a resolution of 1920×1080 at 100% in the first screenshot. To demonstrate the effect of changing the percentage I have set it to something very low like 10% in the second screenshot. While the preview keeps the same display size, the resolution is reduced drastically in the second image:
If nothing of this helps with your issues then it's probably best if you upload your file to invesitgate further what is going on there.