Basically, mixing a 4K screen and a 1080P screen doesn't work properly. On a stream, @Jason Carr mentioned I should take it to the forums and report it, so I have. What happens is BB actually displays BOTH monitors content on one monitor. If you switch both to be 1080P in Windows, reboot while it is set to 1080P it will work. Then you can actually return one of the monitors to 4K and it will actually work properly. If you have either monitor set to 4K before you reboot then BB will not show anything spanned across monitors.
Thing is if you trick it in this way it DOES work properly! This tells me there has got to be something that can be done to make this work reliably. I don't really care to swap back and forth to 1080P and reboot every time I want to turn on my second screen with BB.
Thanks for you time!
Below is a video showing exactly what is happening. The video includes both monitors. The right hand screen is a 1080P 55inch TV so that is as high as it goes. The leftmost screen is a Vizio e65-01 which is a 65inch 4K TV. They are side by side in my setup.