Yeah I just set the desktop resolution to match my actual desktop resolution, on my HTPC it's set to to 1920x1080. Yours should be set to match yours.
I know I have encountered your problem before but I can't remember what exactly caused it, I wonder if maybe you got some sort of scaling set in your Windows Display settings since you are on a 4K display and that may be causing the issue. Not sure at all, just throwing something out there for now.