The trouble with 4k (or a TV with resolution of approx 3840 x 2160 or 2160p (as opposed to 1920 x 1080 or 1080p) is an inconvenient problem commonly called the human eye. There is a physical limit to what it can see at 20/20 vision. Luckily for marketers there are two parts of the brain (lust and stupidity) which trump this inconvenience.
Lets take the "common" 55 inch (disclaimer: size of my 1080p TV) as a bench mark. To benefit from the 1080o definition the ideal sitting distance is 2.1m or less. For your friend with their new 2160p they need to be 0.9m or less away. If you are sitting 4m away then to see the benefits of a 2160p you would need a 210 inch. telly. If you sit further away than these distance the eye simply cant pick up the extra definition.
Now we have the issue of TV size dealt with the next is content. Go to your local shop and look at how much UHD content is available, None. There is some HD BluRay but not a heck of a lot.
But plastic discs are obsolete to so we need to look at streaming services. At the moment on good old fashioned broadcast TV there is sweet bugger all 1080p content. If you have a Tivo just do a search for HD programmes and you'll see there aint much there.
Ah - but there is Netflix I hear you say. Well it streams 4k at around 14mps (which is a whole lot less than Bluray1080 at 48mps) which means you are getting content with substantially sacrificed audio and colour quality
But luckily for me the two other parts of the brain trump the eye. So people are on the 4000k bandwagon which means SKY can trumpet these advances and dimwits think its is a great thing and the share price goes up.