A new set of Linux GPU driver patches have emerged for allowing HDMI 4K support.
The set of new patches were written by Intel's Damien Lespiau and allow for parsing a dark corner of the EDID information for being able to expose the 4K x 2K resolution modes to user-space. These ultra high-resolution modes are supported by the HDMI 1.4 specification.
Besides needing to parse the EDID information for the 4K x 2K modes via the HDMI VICs in the HDMI CEA block, there's a currently queued up Intel i915 DRM driver patch for supporting a 300MHz TMDS clock and an already-merged xf86-video-intel DDX driver patch needed for preserving the EDID blob for the lifetime of an output.
Unfortunately, 4K x 2K support also requires not-yet-merged X.Org Server patches for using the TMDS maximum frequency to check the mode dot clock.
Intel's hardware that support HDMI 4K resolution outputs in conjunction with these various open-source Linux graphics patches is the latest-generation Haswell processors.
More details on these patches can be found on the intel-gfx list