How to properly use scaling on an external display (from MacBook Pro 2016)? I have a MacBook Pro 2016 (with Thunderbolt 3 ports on macOS 10.13.6) and I need an external display. 
I bought an ASUS VP28U (4K), connected it with an HDMI to USB-C adapter and, generally, it works. The problem is that everything is super-small, I guess because of the 4K. 

Why doesn’t this display have any scaling option? 
or...
How can I adjust the scaling of an external display while still using the full 4K resolution quality? 
The only options given to me were to reduce it to 1080p, with obvious results...

Am I making something wrong? Using the wrong cables? Missing some options?

Or is the display a bad one?
Anyone with experience with my same configuration may help in sharing his setup?

Thank you
You need to set it to 1080p in order to get the scaling. A bit counter-intuitive this actually means that the display is using the full 4k resolution, but font sizes, window decorations, etc. is scaled to have the same size as with a 1080p display. This is what Apple calls "Retina Mode": Video, photos, etc. will display in their full resolution.

So try setting it to 1080p and then check on your monitor's on screen display that the incoming signal is actually 4k!

Note that the list of resolution in System Preferences > Display will also contain resolutions such as "1920 x 1080 (low resolution)". The added "(low resolution)" means that these are actual 1920x1080 resolutions, and not scaled like in Retina Mode.

If you do not see the list of resolutions, then hold down the Option-key and click the "Scaled" radio-button besides "Resolution:".