I'm currently reading Color Design Workbook: A Real World Guide to Using Color in Graphic Design, but it's a bit short on color theory basics. I was wondering: in CMYK, black is added to the three subtractive primary colors to improve the rendering of blacks, which would otherwise be brownish. This addition of a fourth pigment increases the gamuth of available color.
So, my question is: has there been any serious proposal to add a fourth primary to the additive RGB colors? One can imagine that this would allow for more vibrant non-primary colors, for example in the “violet” region, in high-end display devices. One obvious downside would be the price of such new technology, and I know it is not widely available, but: has it been considered?
Yes, it's been considered and manufactured. One such technology is the Quattron display technology from Sharp. This technology uses a fourth yellow sub-pixel which
"increases the range of displayable colors, and which may mimic more closely the way the brain processes color information".
Since the cones in your eyes have their greatest sensitivities to red, green, and blue, those three colors should be sufficient to display any color. It is not a problem with the number of primary colors, but the range. For example a typical computer display has 256 levels of each color, the display is limited to a little over 16 million colors. If you want more vibrant colors, then increase the number of discrete steps, not the number of primaries.