If you're using 4 of them versus 3, wouldn't this require 1/3 more subpixels to achieve the same display resolution in pixels?

Or -using same # of subpixels per cm^2- would perceived display quality be similar due to better color representation?

Probably yes, but density hasn’t been an issue for over a decade. Mobile device screens are 5-10x denser than TVs, and cost <€100; Apple has used mini-led for their retina displays for a long time too.

Yes, it would require more. No, at least assuming the pixels aren't well below the eye's spatial resolution limit then resolution will impact perceived quality much more than color accuracy. Consider what lossy image codecs like to do to chroma and why that is.

However, there are tasks that benefit from better color reproduction. There are also screens where the pixel size is well below the human discernable limit.