So there are R, G, and B cones. The signals from these cones don’t go straight to the brain; they first pass through a pre-processing filter, and it’s this filter that explains all the mysteries. Actually there are three filters.
To do this “wheel” thing properly, you have to represent the red/green and blue/yellow opposites. It’s not at all difficult, so it amazes me how rarely it’s seen or taught:
Four primary colors? Yes, why not? It’s the closest thing to the actual physiology without getting complex.