"Sharp Quattron’s fourth primary color is yellow, and there is nothing for it to do because yellow is already reproduced with mixtures of the red and green primaries, he said."
This 20-year display "expert" has never heard of hi-fidelity color? Inexcusable. It is accurate to say that yellow is not a primary color of light, so it is true enough that no one should refer to the yellow pixels as a "fourth" primary color. However, it is completely false to say that "there is nothing for it [the yellow pixels] to do". The yellow pixels are used to extend the gamut of the device to cover portions of the visible spectrum that the primary LEDs, in combination, cannot.
We do this all the time with Hi-Fi printers. We add RGB or Orange, Green, and Violet to the traditional CMYK primaries to expand the gamut of the device far beyond what traditional process color can accomplish.