I have a similar setup and discovered that when I ran in full screen mode on my 32'' 4K monitor the frame rate dropped and I found it hard to click on the options because they were so small. I don't know if it's a solution to the problem, but my work around was to run in window mode and expand the window to fit the size of the screen which was pretty close to full screen mode. I been running for almost a year with that workaround.