Magic Lantern has released a build of their firmware that offers 4K RAW recording on the Canon EOS 5D Mark III. Although still experimental, it offers the following recording resolutions:

• 1920x960 @ 50p (both 1:1 crop and full-frame - 3x3 pixel binning)
• 1920x800 @ 60p (same as above)
• 1920x1080 @ 45p and 48p (3x3 binning)
• 1920x1920 @ 24p (1:1 square crop)
• 3072x1920 @ 24p (1:1 crop)
• 3840x1600 @ 24p (1:1 crop)
• 4096x2560 @ 12.5p (1:1 crop)
• 4096x1440 @ 25p (1:1 crop)…

Also within the build is a Live View mode that shoots at 5796x3870 at 7.4 fps, even at faster shutter speeds. This will produce rolling shutter however. Magic Lantern point out that the current build has “plenty of bugs,” but expect that these will be worked out over time.