I'm a big XFCE fan but multimonitor support is still iffy, especially if one is a HDPI one and another not, as the scaling applies to all monitors.
I'm not even sure what the point of HiDPI is though. It never made a difference for my work as a web developer. I understand it's more pixels per inch and less pixelation for high-res images, but if everything is getting scaled up what's the difference?
My (Windows 10) gaming rig used to be hooked up to a 4K 50" Samsung TV but I think the max I could get was 60Hz. Games and movies look pretty good there, but eventually I switched to a cheap 1080p/144hz gaming monitor because I play Rocket League mainly and that is a fast action e-sports game where you want to see as many FPS as possible.
On a 1080p I can easily see individual pixels from a mile away and everything looks simply bad. I can not stand text on a low dpi screen.
Also, when I use 1.5x scaling, I have 1.5x more real estate. Hence my requirement for the fractional scaling. Even on 2x scaling (effectively turning it into a 1080p display) everything looks much better than a native 1080p display.