I'm not entirely sure what I'm about to say will help answer your question, but I'm in monitor mode right now after picking up a Samsung U28E590D so I'll reply anyway.
I've never used a 4K monitor before and after recently picking up a 4K TV and needing a monitor, 4K made a modicum of sense. Plus, I've always liked having a shed load of space on-screen for stuff, and being a Mac user I've fallen into using everything full-screen like macOS loves to do. I wanted out of that and to be able to have multiple pages open at once - comes in useful for the job, so, I bought 4K.
The monitor arrived yesterday and sure enough macOS found it as 4K and shoved a full 4K image at it, unscaled. Now, I like to think I have decent eyesight but my god, things are small. Too small, I'd argue, unless you're using something like video editing software and need to be able to see a 1080p stream and have a load of controls etc on-screen at the same time. But for text and stuff, no way.
So the outcome is that I've got macOS running in 4K but scaling everything to what it claims is the equivalent of a 3008 x 1692 display and it looks gorgeous. Lots of space, everything's sharp as a sharp thing and we're good to go. Apart from the stuck green pixel that hasn't decided if it's going to stay stuck or start working yet, but that's another issue.
TL;DR - I think I recommend getting a 4K and letting the OS scale something between 1440p and 4K. I assume Win10 does something similar?
