Comment 10 for bug 1687246

It's working for me in Ubuntu 17.10 - gnome-settings lets me choose scaling between 100% and 200% in 25% increments for each monitor separately (and it defaults to 200% on my 4K screen with my 1920x1080 screen attached). You might have to change a setting for it to work - see http://www.omgubuntu.co.uk/2017/09/enable-fractional-scaling-gnome-linux.