I had the same monitor and had the same exact issue and then i tried some other monitors capable of 4k and same black screen..what fixed the issue was inside Dev folder in System, there is file called Radeon HD 6450..right click on it and go to info and make sure under icon tab that the resolution is in there. I had to this with an older monitor that only supported 1920x1080. Once entered in the correct resolutions, everything worked on thereafter.
