This piece over at EOS HD New Sony RX10 sensor has 5K full pixel readout caught my eye (it references the review over at ImagingResource.com, but is more to the point on this topic).
Essentially it says that for 4K video, the entire 5472 x 3080 sensor is read-out at 60 fps, then downsampled to 4K resolution. This is the right way to do it, but generally not the way it is done with most all cameras, since it requires very high CPU processing power.
What caught my attention is that it dovetails with mangled Live View on the Nikon D800/D800: the subsampling of every 3rd row explains the pixellated staircasing of Live View quality at full magnification: the D800/D800E are not reading out the entire sensor, even at 10X magnification.
The Sony RX10 uses a RX100-size sensor of 20.2 megapixels in a DSLR-style form factor along with a 813g weight and 24-200mm (equiv) zoom. It’s massive compared to the RX100, but might be a good choice for some travel shooting. About $1298. I’ll be reviewing it later this year when it becomes available.