It would help with saturation. Yellow doesn't have much space between red and green. If you look at the typical color space triangle and add a 4th pole at the edge in yellow, it doesn't add much. Less so with wider color spaces, but there is apparently an issue with perception of yellow and it's level of saturation when the red and green primaries get farther apart (and I don't have a clue how to explain it, has to do with photoreceptors in our eyes I think). A yellow primary would help with that. But you would need the entire chain starting with the capture device to properly support it. There's no way to correctly add that additional yellow information outside of the existing color space to an existing RGB signal. There's also the effects of the transition from green to yellow and yellow to red along the edges which can't be added correctly.
The problem with the Quattron was that it was still using an RGB signal, so to make yellow it was still mixing red and green. The yellow subpixels only acted effectively as enlargements of the existing red and green subpixels, allowing additional red and green light through which could then be perceived as more yellow. I think there was some additional processing going on that tried to raise the saturation of the secondary colors, but it just ended up messing with accuracy.