Game mode in monitors is almost always useless because as you said, it creates artifacts around text and other contrasty scenes. In game mode, monitors try to minimize latency and blur by pushing the panel beyond its limits. It’s usually better to stay with the “normal” monitor profiles.
It’s true that many large 4k monitors have features that let you divide the screen in 4, which mimics having 4 separate 1080p monitors, but that’s mostly a gimmick. As you said, everything looks extremely small.
The real advantage of 4k is that once you set up Windows text scaling properly, you can’t tell the pixels apart because they are extremely close and extremely small, that makes everything look amazingly sharp (especially text and movies).
If you ask me, games don’t benefit that much from 4k because the performance hit is significantly worse than the sharpness improvement you get with the extra resolution (of course if you have deep enough pockets you can have the cake and eat it too). FS2020 with its best antialiasing looks good enough in 1080p. I did buy a 4k monitor but fully knowing that I will be switching constantly between 4k and 1080p depending on what I’m doing.