We have rods and three types of cones, which look like four channels to me. In very low light the cones don't fire and we see in black and white. In reasonably bright settings I can imagine that the rods saturate and we only have three useful channels. But there must be a level of light where both cones and rods are within their dynamic range. What am I missing?
I am not sure if human vision is using one or the other. When both rods and cones are within the dynamic range, possibly response from both get utilized by the brain. I am not sure.
Do you have a reference for this? Quick googling seems to show that the rods' response curve is not, it seems to be a most sensitive at a wavelengths between the peak of the blue and green cones.
"More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions." http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.ht...
So while rods have a response curve which is similar to codes, that response curve is shifted to be a thousand times more sensitive than cones. So at any lighting level which activates the cones, your rods are just maxed out, screaming "YES, IT IS VERY BRIGHT", and you don't get any useful information out of them.
Hence, no integrated, tetra-chromatic vision.