[truncate]

I owe a Dell XPS 13 with 4k display running Linux. Fonts are clearly much more sharper (maybe even feel darker) and pleasant to read than 1080p or even 2k . Although, I see you point. For most practical purposes it does not make difference I think, but I really love 4k when I'm programming. These days I'm not programming much, hence switched to 1080p as little experiment (and got fed up with sloppy GNOME performance), and I would say I don't notice much for general tasks.

[Tepix]

There are two generations of the Dell XPS 13, at first they had a 3200x1800 display, they switched to 3840x2160 last year.

In my opinion 3200x1800 at 13.3" is more than enough already (I own one) but I guess marketing wanted more.

[truncate]

Yes, I also have the old 2015 model (3200x1800). Definitely, good enough in my opinion too.

[Brakenshire]

Are they in the range where the UX size can be doubled without being too large?

[truncate]

Yes. 2x worked perfect for me at 4k, and its the default for hidpi settings I think. When I switched to 1080p I had to manually increase font sizes (not overall scaling) because the default size wasnt sharp/readable enough. Its pretty cool 4k now works so well out-of-box on Linux. It was nightmare in 2015.