"Theoretically, one could put another pixel between every two active but it is not possible in practice"
Doesn't work like that. Probably you don't know that a CCD has an array of microlenses in order to use almost all the area from pixel to pixel. The effective active area of a Pixel is indeed smaller but almost all the area is used without space in the middle in order to catch light thanks to the microlenses. THis is true for a single CCD or 3 CCD and doesn't really leave "space in the middle" unless is done intentionally but in that case some light is lost with all the consequences......

Only in the Sigma SD9 and in the Kodak 14N photo cameras there weren't microlenses since microlenses can produce aberrations in certain conditions (using some old optics) . Both cameras have ben discontinued and reintroduced in the market with the layer of microlenses.

<<<-- Originally posted by Andre Jesmanowicz: Is it clear now? -->>>

Very clear, as it has been.
Remember that no camera uses 4ccd construction.
If you are using pixelshifting with all ccd's, resolution with this 4ccd system would be identical to 1ccd system, that uses same amount of silicon. Eg. 4x 1/3" 1Mp compared to 1x 2/3" 4Mp.
What is the benefit of having 4ccd's then?
You want to have deep DOF?

When digital cinema will reach its prime (after some decades) there might be a need for multi imager systems again. If the imagers are already big enough and one would like to have more resolution and sensivity but not less DOF.

I want nothing. It was simple explanation of how one can get increased resolution by shifting a green chip in 3 ccd cameras.