Most likely your 4k monitor is 16:9. The resolution you picked is 1920:1200 which is 16:10. Try picking a 16:9 ratio like 1920x1080 or 2560x1440.
Yeppers you hit it on the head. Changed to 1920x1080 and it at least filled my screen. But I really miss the missing 120 pixels hahaha