Don't judge a book by its cover
In this case the cover is the TV. Whether it is 4K, 3D, 625 lines (yikes!) or even in colour.
The "book", however is the content: the programmes, video games, films and is (hopefully) the reason why we bought the telly, in the first place. To watch them, not to dribble lovingly at the sight of an enormous screen.
Now, it does seem that with every new revision of the "cutting edge", the technology improves but the programmes (and other stuff) do not. So we end up spending £100's or £1000s on spiffy new kit, only to turn it on and watch 40 year-old episodes of Dad's Army.
Unless there's something completely immersive about 4K, and there is sufficient content made available specifically for it then there seems little point in upgrading - at least until the currrent telly has worn out.
There does seem to be one aspect of 4K that would make it a "must buy": the ability to get three or four ordinary TV programmes running as P-i-P at the same time. Then we might stand a chance of finding something decent to watch. Provided the picture quality isn't compressed to hell and back, just to get a number of channels down the satellite link.