Different saturation between the development modul and fullscreen preview in Lightroom CC

First of all, try to disable the GPU. Go to Edit > Preferences > Performance, uncheck Use graphics processor.

If  this fixes the issue, update your graphics driver, then try turning the GPU back on. You probably need it with a 4k monitor.

If that doesn't help, you most likely have a defective or incompatible monitor profile.

If you have used the Palette Master calibration software, that may be the cause of the problem.  There have been numerous reports about it creating buggy profiles. Because of this new forum's useless search function, I'm unable to find any of these threads.

To troubleshoot the monitor profile, try setting it to Adobe RGB. 

Press the Windows key + R, type colorcpl in the box and press Enter, which will open the Color management dialog.

Add Adobe RGB, then set it as default. (the screenshot below uses sRGB, you should use Adobe RGB since you have a wide gamut monitor). Lightroom needs to be restarted to become aware of the new profile.

If this fixes the issue, try recalibrating with Palette Master (make sure it's up to date), making a version 2 (not version 4), matrix based (not table based) profile.  Alternatively, use the software that came with your calibrator.

Hi Stefan

Did you calibrate your monitor? Since this is a wide-gamut monitor, applications need to be able to call a matching profile.

Franz