I have a 4k monitor, and I am also using the MBP display. I have the 4k set on
Scaled with the middle option. The following happens with windows that are open on the 4k monitor (it does not happen with ones on the MBP display).
When any window is already resized using the built-in resize options (Left/Right third/two thirds), moving it to the next monitor does not work. Instead of moving to the next monitor, the window positions itself partially off the screen, on the bottom left.
Screen capture: http://www.giphy.com/gifs/gFnLr0pT4AM4IFiasB
Edit: The alpha build from today does not solve the problem.