I've read on HN before that Linux has shaky HiDPI support but after I setup my new computer and started looking into it I got really worried as my research indicated I am in trouble. I have 1070ti nvidia card, 3 older 1080p monitors and beautiful new 4K LG monitor.
It seems X server does not support different dpi setup and using Wayland is not possible because nvidia. However actually I managed to solve it quite simply. I have not tested it fully yet but maybe it will be helpful to someone too.
1. I am using Kubuntu so I went to KDE -> Display configuration and set 200% global scale. Now my 4K monitor is fine but everything is very large on 1080p monitors.
2. Do xrandr --output DVI-D-0 --scale 2x2 on all 3 1080p monitors.
3. Go to NVIDIA X Server Settings (I installed NVidia proprietary drivers) and align all 4 displays as I want (they all should be 3840x2160). Also check Force Full Composition Pipeline on all 1080p monitors. Go Antialiasing settings and uncheck FXAA (had some issues of blurry text that is out of focus).
Apply. Save to X configuration file.
4. Restart.
It worked! I was so excited that I had to post it here :)