What is the physical size of your 4k monitor? Is it a 27" monitor, a 42" monitor, etc? This would be the diagonal measurement, corner to corner. I'm working right now on my built-in laptop screen which is 17". It's the combination of 'resolution' (your '4k' terminology, or my '2560x1600' terminology - same thing) and physical size that leads to issues. My '2560x1600' resolution is barely readable on a 17" screen, but I have font-scaling set to 150% so that helps.
Ironically, buying a 'high res' monitor that has small physical dimensions is a recipe for disappointment. For the longest time, I wouldn't go above 1920x1200 on any laptop due to poor scaling support. Only recently has Windows finally built in some really good scaling support, and that's why this new laptop is now 2560x1600. But, as noted, Quicken and Photoshop are still not very usable - they worked much better when I had a 1920x1200 screen ("FHD" I believe is the buzzword for that res).
One thing I've noticed - programs that work on both Apple and Windows platforms seem to have more of an issue with this. I'm guessing that the way Windows handles scaling is totally different from the way Apple handles it, and if the developer wants to maintain a common code-base, the Windows display suffers. How else can Photoshop - which is an extremely expensive and 'high end' product - have such a hard time dealing with font scaling on Windows!
In Photoshop Elements (I can't justify the cost of the full Photoshop these days), they have a feature where you can change font sizes in the UI but - 'large' (200%) is ridiculously large (almost cartoonish), while 'small' (100%) is too small. For some reason, they can't provide a user-defined scaling factor, and ... I guess offering 150% would be just too much work!
This is Photoshop Elements 2023 at 100% scaling - the menu bar at the top is virtually unreadable on my screen:
And this is the EXACT same screen in PE at 200% scaling - the menus and fonts are so big, there's no room left for the actual image!
I don't think this is off topic at all, though - my guess is, the developers of XnView are running into the same issues that Photoshop is running into; they are using some libraries that are common between MacOS and Windows and they just don't handle scaling well. If Adobe can't fix this, not much hope for a freeware program like XnView perhaps. I hope I'm wrong! Actually - for me - XnView is MUCH BETTER than Photoshop in terms of UI scaling; but I think in your case, your monitor is such high resolution that you are inadvertently suffering.