You didn't understand the problem mentioned by GP. Neither of your monitors requires HiDPI scaling. Your first monitor has approximately 138 dpi and the second a paltry 94 dpi.
Compare that to a 2012 MacBook Pro that has a 15.4-inch display with a 2880x1800 resolution. That's 221 dpi. The hallmark of a HiDPI is that its DPI is so high that you need to render UI elements at 2x or more (though sometimes 1.5x also works), so that originally a one-pixel line becomes two pixels wide. This requires support not just in drivers and the OS, but the application. ArchLinux has an entire wiki page discussing application support: https://wiki.archlinux.org/index.php/HiDPI
I beg to differ, 4k/32 does need scaling to my eyes. I don't want things to be that tiny.
Not every application needs to support scaling, so long as the GUI framework they use does, and the window manager does (XFCE has support, which I have activated) that makes the 4K screen more usable to me (while allowing the full res to be used for smooth rendering), then I use xrandr to scale the lower-dpi screen back down so elements are the same size across both. Sure, there are a few apps that don't behave brilliantly, but they aren't things I use much.
Like I said, it works better for me than MacOS or Windows (I have both, and a 2103 Retina MBP I still use).
For stuff like this, Linux is bullshit. For many desktop usecases, Linux is bullshit. After a Skype call I need to kill the app else the process takes my CPU usage to 100%. Why? God knows. Probably the same reason my lock screen sometimes shows a password prompt when I press 'Esc' or 'Enter' and sometimes it requires me to click the mouse. Did I mention I spent the last nine months without a working microphone because the drivers for my new Thinkpad hadn't made their way into any major distribution??
Don't get me wrong. I love Linux. I'd take its broken ass over Windows any day of the week. But fuck me if I don't miss macOS.
Pretty sure it doesn't :) That '1200p' screen I have certainly isn't in need of scaling for usability, but I have scaled it to match the size of screen elements on my 4k screen next to it.
I've been going back and forth between MacOs and linux a lot lately, been using my Mac as my work laptop for the last couple of years, now that I'm working remotely 100% I figured I'd switch over to linux. Honestly not having much issue with it, bar the odd "Teams has decided it can't use your headset microphone, for reasons" error.
Never even heard of Dia, let alone had call to use it. Just installed it and yes, for unknown reasons it doesn't seem to scale does it? Which is odd given it's presumably a gtk3 app and others do scale just fine. Looks like GIMP is another one that's not onboard yet.
My daily use at the moment is IntelliJ, MS Teams, Skype, Firefox and Steam. I guess gedit, meld and a few other utilities. They all seem to be fine. Perhaps my relatively constrained set of requirements is why I'm happy with it.
I'm sitting here with a 4k 32" monitor and a 1920x1200 24 inch side by side, scaled perfectly so both are pretty and readable, in xfce, and when moving things between screens nothing changes size, or re-renders weirdly, or can't be resized due to invisible boundaries, or ...
It's better than windows or MacOS can manage with the same setup.
I'm not sure what you count as 'proper', and yes I had to write a two-line script with xrandr commands so ease of use isn't massively present. But the underlying system absolutely has great scaling support.
Graphics drivers are also fine these days, I've not had any problems with my mouse either. Hardware in general "just works" better than windows these days, with older devices supported and no drivers to hunt down on the web.
I'm not trying to claim it's better for you, only you can decide that. But it works great for a lot of people.