How to increase system default font size under macOS Mojave for 4k display

I have connected my MacBook Pro (2018 model, 15'', running Mojave) to a 4k display (Asus 28'' via HDMI via USB22HDMI-Adapter) and the result is that I can either use it in highest resolution with tiny default font or discard the 4k resolution and make the font readable.

Is there a way to increase the default font size (say for all apple APPs/cocoa APPs)? The best would be to make that choice per display, i.e. the 2k retina display should remain with the default font size while the 4k display should scale up.

Adjust your display

Choose Apple () menu > System Preferences, then click Displays to adjust or scale the resolution on your display.

Using 4K displays, 5K displays, and Ultra HD TVs with your Mac - Apple Support


 https://support.apple.com/en-us/HT206587

The only way to increase the font size is to change resolutions.

Hold down the option key and click in Scaled. It should offer more options for your external display including some HiDPI options. These “retina” resolutions should work just as the internal display in a retina mac. For example the default setting in a 2880x1800 retina display in a15” mbp is nominally 1440x900, so the font is not too small, but gets the added detail and can show you 1080p video pixel per pixel.