OK, so your monitor is 4K, so native resolution is 3840x2160 and this is non HiDPI resolution. All others (except those marked “‘low resolution” are HiDPI ones. Which one are you using now?
I have it set to the default which is 3840x2160. Does that mean I should change the resolution to something else? Thanks again for your help. I'm having trouble understanding what resolution should be used.