ASUS Zenbook Pro UX501VW

Xorg
For proper input, video, and high DPI support on the 3840x2160 screen:

Create a xorg configuration file to set the monitor size in millimeters (these values were quickly and crudely calculated; you may want to measure your own):

If not using GNOME or some other software which manages Xresources and DPI for you, you will need to specify the DPI:

And you will need this line in someplace like or  (depending on your configuration) in order to apply the Xresources file:

Other applications may handle high DPI weirdly. The Arch wiki page on High DPI has many helpful hints.