2K Proxy ! that the only way to edit 4K .
There lots of reason to have 4k material , but from my Video that's posted here Reply #9
I made it very clear it was just a test to see if it could work.
And it dose , if you need a few seconds of 4k it could be achieved , but only in 3xCrop mode at least for the 5D2.
As far as monitoring 4k in your NLE or Grading app , most if not all app will do a scaling option for your display .
The biggest benefit to have 4k over 2k,HD material is the possibly of reframing the shot .
I just when thought this on a film I just edited , shot on a bmcc 2.4k raw output was 2048x1152 , 2:1 A.R.
and I was able to use a few shot over a few times by just reframing

or the simply fact you can punch in to a shot as if you zoom in .
There so many advantage's to that .