You may be hitting the same issue as I. 4K screens run in tiled mode requiring the MST mode of Displayport (DP 1.2), so your display acts as two 1920x2160 displays and the driver makes it appear as one. Which MBP Model do you have? The current one is available in three varieties if GFX is concerned... discrede nVidia GT 750m, intel Iris Pro, and the regular iris GFX for the 13" incher. nVidia does support tiled mode (depending on driver and card), and as far as intel is conerend, there are conflicting statements - the Iris Pro Models should work, but if the lower end models work is up for debate. The data model for an HD 4400 for instance (same as I have), states that the max res/refresh rate is 3840x2160@30Hz. However, the datasheet also says that it can support two displays of a resolution higher than 1920x2160 at 60Hz, meaning there's enough pixel bandwidth for a single 4K screen. Yet, when I last tested (January), using the collage mode (required to sticth the two halves of a 4K display together) would lead in unpredictable / unstable behavior. I'm currently waiting for Dell to replace my 4K screen with a version that has the updated firmware that's supposed to fix all issues with nVidia displays.. then I'll give the HD4400 another go.
Have you tried activating the collage mode?