You are probably familiar with the horseshoe-shaped chromaticity diagram [0] of human-visible colors. A light source with three color primaries spans a triangle in that coordinate system. To cover the whole horseshoe, at least one of the vertices would need to be way outside the horseshoe. With four color primaries, you get a quadrilateral that makes it easier to cover a larger portion of the horseshoe.
The reason the visible colors form a horseshoe rather than a triangle is due to how the cones’ sensitivity ranges overlap [1]. They cannot be excited independently by the primaries of a display.
The reason the visible colors form a horseshoe rather than a triangle is due to how the cones’ sensitivity ranges overlap [1]. They cannot be excited independently by the primaries of a display.
[0] https://upload.wikimedia.org/wikipedia/commons/1/1e/CIE1931x...
[1] https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Co...