config dual monitors with quite different resolution

if you're using Gnome on Ubuntu, you can use xrandr command to make screens different from one another. For example:

xrandr --output HDMI1 --scale 2x2 --mode 1920x1200 --fb 3840x2160 --pos 0x0
xrandr --output DVI-0 --scale 1x1 --pos 320x2160

--scale 2x2 - make everything on 4K screen twice larger

--mode XxY - explicitly set the resolution for the screen (not necessary if is already set)

--fb XxY - set the size of a virtual screen (framebuffer) (this iss very important. Without this, you will be able to use only a fourth of your screen). That maximum framebuffer size might be specified in xorg.conf - then you cannot exceed it (it is written in the first line of xrandr -q output).

--pos XxY - in my case I set the absolute positioning of the screens, so my laptop screen is directly on the bottom of the external screen.

sources: Arch Wiki Page and This post