Another issue with the discussion above that it isn't quite literally like having 4 x 1080p screens, because the pixel density is likely to be different between the 1080p display and the 4k display.
What I was saying there is that a 4k literally has 4 times as many pixels as a 1080p, which means that it's effectively like a 2x2 grid of 1080p displays, which does indeed mean that if you use 100% scaling everything looks ridiculously small, particularly if you have been using a 1080p display for a decade. It's literally like you took four 1080p displays and shrunk them down, and put then in a 2x2 grid (without any seams, obviously.)
And what prompted me mentioning that is that, indeed, when I got my first 4k display, Windows used 150% scaling by default, but I did try the 100% scaling out of curiosity, and everything looked ridiculously small. It felt unusable.
However, I later found out that it's just a question of getting used to it, and in fact you get used to it really fast. I mean, you just need to use your display with 100% scaling on Windows for a day, and you'll likely have been used to it by that point, and you'll very unlikely will want to go back to any larger scalings.
Since then I have thought of going back to even 125% as unthinkable. The small text in everything is super-sharp and very readable, and it doesn't bother me in the least. But I have absolutely fallen in love by the
enormous amount of desktop space that it gives me. You can't fully appreciate how much it is until you have used it for a while.
I have been using my old 1080p display as a secondary monitor (eg. to run EDDiscovery there while ED itself is on the main 4k display), and it feels super-cramped in comparison. I can't fit almost anything there. What takes literally like 20% of my 4k display almost fills up the entire 1080p display. I literally am able to run one app there without overlapping. How I ever lived so long with a 1080p display, I don't understand...
(It doesn't exactly help that the 1080p display is a really old and cheap TN panel, which not only makes the picture look blurry in comparison, but also really low-contrast and washed-out compared to my IPS 4k display. Yes, I have tried improving the contrast and saturation of the old display, but it doesn't help much. It's just horrible.)