In short, 4 million pixels carry all of the useful information that you can put into an 8' x 12' photograph. Finer detail than this will matter to technical aficionados making magnified comparisons, and it may matter for scientific or forensic tasks, but it will not matter for ordinary purposes. The same holds for larger prints because we don't normally view larger photographs from only 10 inches away. It holds even for the gigantic images in first-run movie theatres. The digital processing used routinely for editing and special effects generates movies with no more than 2,048 pixels of information from left to right, no matter how wide the screen. The vertical dimension differs among cinematic formats but is typically around 1,500 pixels.
Ok, I'm convinced. In practice 8 megapixels would be great since it gives room to crop still print 8x12.
I want an 8 mpixel sensor with low noise at ISO 1600 and excellent color range.