I don't have a 4K monitor, but what does `xrandr --output HDMI1 --scale 0.8x0.8` do? I have a 1024x768 monitor and do to all the useless whitespace in modern programs, I scale into the opposite direction.
But I agree Gnome lost the plot completely, and sadly Gtk too. Which is a pity, because I prefer GTK+ to Qt, but they deprecated so much useful Widgets and the alternative given is 'just don't do that'.