first off _ i have designer pro+; the designer 18 implementation of 4K support has not afaik been documented as different, but who knows for sure outside of xara/magix

I have not noticed anything in the xara UI that is not consistantly very small at native 4K, no scaling

long shot, but you do have graphics hardware on your computer capable [for multiple monitors if you use them at the same time] and all the drivers are up to date....