OK, so I got 4k "working" on my UY04. The fix is bizarre. I have a new XPS 13 (2015) connected via Mini-Displayport to HDMI. Out of the box, the system only outputs 1080p, with no higher resolutions available. But when I *uninstall* the Intel Graphics Driver, and reboot using the Microsoft Basic Display Adapter, I can see full 4K resolution.
I was told by Dell's Technical Support that Intel's graphics drivers do not yet support Displayport 1.2, so they are limited to HD resolution on the external monitor. (!)
They are incorrect (Dell support).
The reason I can say that...is that my laptop is, wait for it...
A 2015 xps13 (9345) i5 8gb 256
It worked with fine with HDMI to miniDP and Intel drivers with my uy04...but not my UY04-1.
Has to be a firmware issue...because its not the driver package.