Is RED 4K really 4K? There is a really good interview of John Galt, Panavision Senior Vice President, Advanced Digital Imaging at creativecow.net and here from few good quotes from the interview.
Any of the high-end high definition video cameras, they had 3 sensors: one 1 red, a green and a blue photosite to create 1 RGB pixel.I dug a bit deep into this subject and though I felt it was too technical but I got some good insight on this subject.
But what we have seen particularly with these Bayer pattern cameras is that they are basically sub-sampled chroma cameras. In other words they have half the number of color pixels as they do luminance And the luminance is what they call green typically. So what happens is you have two green photo sites for every red and blue.
So how do get RGB out of that? What do you have to do is, you have to interpolate the red and the blues to match the greens. So you are basically creating, interpolating, what wasn't there, you're imagining what it is, what its going to be. Thats essentially what it is. You can do this extremely well, particularly if the green response is very broad.
Well 4K in the world of the professionals who do this, and you say “4K,” it means you have 4096 red, 4096 green and 4096 blue photo sites.
To really understand this issue first we must take a look at what really defines quality of the image. A common misconception is that resolutions increases the sharpness of the image but in reality the fine details are mostly irrelevant to human eye, though human eye is capable of reading extremely fine details. This is also true at greater distance and what really defines the sharpness of the image are the contour defining features at a higher contrast. It is important to note that above statement doesn't mean a low resolution image with high contrast will give better viewing experience since it could lead to aliasing issues.
Since resolution and sharpness are completely separate terms, we establish relationship between them using MTF (Modulation Transfer Function). So each time when generation loss happens during transfer of medium we define it using MTF.
There is a lot more to read and understand on this subject. A good start on that would be the main source of information of what I have written here, an article called 4K systems by Dr. Hans Keining, R&D Depth ARRI .
To conclude, counting pixels wouldn't be the best way to judge quality of an image and the quality of an image depends upon lot of attributes. The MTF just doesn't start at post production, for example there are modulation happening within the camera during the capturing process. So it is really important to take a closer study at these issues separately.
Coming back to the RED discussion, I believe RED's subsampled 4K image would be slightly advantageous even in HD or SD (based on a broadcast scenario) production since MTF would be comparatively lesser compare to a normal 4:2:2 color sampled HD camera.