Theatrical 4K is not the same as Ultra-HD, often marketed as "4K UHD". Seriously, don't muddle these up! The linked article did not, it even had "Theatrical 4K" explicitly, being a link to an explanation of the differences.
The cinema standard 4K is 4096*2160, not quite 16:9 aspect ratio. However, movies can be of any aspect ratio that would fill either the width or the height. With Star Wars being in 2.35:1 aspect ratio, that becomes 4096*1743. Pixels are square and there is no overscan.
Ultra-HD, the TV and BluRay standard is 3840*2160 pixels. Some HDTV's do have overscan, not showing the entire picture, by the way.
Cinema 4K also uses the DCI-P3 colour space and theatrical projectors are capable of the entire range of this colour space.
Regular Ultra-HD is not that good. Ultra-HD with HDR uses a larger colour space than DCI-P3 but mainstream LCD panels at the moment are not capable of displaying that properly even if they can handle the input signal.