Microsoft demos a three-display 4K setup running Dirt3 on Windows 8 at 11,520 x 2,160 resolution at 30 Hz, and running Dirt3 on a single 4K Display at 60 Hz using Multi-Stream Transport (MST).
Apparently they had to get a experimental driver from AMD to support the huge resolution.
Full story from microsoft: http://blogs.windows.com/windows/b/extremewindows/archive/2013/07/25/pushing-the-12k-pc-gaming-boundary-at-1-5-billion-pixels-per-second.aspx