With some DisplayPort 4K displays, they expose themselves to the PC as two DisplayPort 1.2 multi-stream devices. These displays having matching GUIDs to tell they are from the same device but different EDID, additionally some information is supposed to be provided for revealing their orientation.
These 4K tiled displays are a problem for the Linux graphics stack and now there needs to be some generally agreed upon guidance for how to support these displays with some applications treating them as completely separate monitors -- e.g. only being able to maximize to half of the display, games not displaying correctly, etc.
Among the options being evaluated by Aaron Plattner would be to hide the presence of the second tile in the X.Org Server, hiding the second tile in the X RandR library, updating all RandR 1.2+ applications for making them aware of tiled displays, or simply doing nothing and hoping that single-tile 4K displays become very common quickly. Aaron has also proposed a new OutputGroup property for the Resize and Rotate extension.
There hasn't been much feedback yet on this call for action about 4K tiled displays on Linux, but you can check this xorg-devel thread for updates. For those curious about GPU / driver testing on 4K displays, you can see my 4k Linux graphics testing from a 39-inch Seiki 4K TV.