I haven’t had the (dis?)pleasure of calibrating one of them (I haven’t seen anyone that actually has any of them in hand yet), but the current thinking among the calibration community seems to be that the 4th color serves two primary purposes:
1) Make the image brighter, thus selling more TVs (you can get more brightness from yellow than green)
2) Make it so they can market the technology to the unsuspecting public as something new and cool, thus selling more TVs
This is second-hand information, so it needs confirmation, but apparently they are cutting the size of the green pixel in half in order to get a half-size yellow pixel. This will result in greater overall brightness of the image (especially white).
As for “more colors”, that is marketing verbiage since the rec709 specification says what colors are part of the broadcast standard. Most/all current LCD TVs can reproduce just about all the colors in that spec, especially when calibrated. They might artificially enhance yellows to push that secondary outside the rec709 triangle, but of course we’d bring that in when calibrating.
