Popped into John Lewis this morning to look at the 4K screen. The 4K was sat directly next to the 5K, and I explained to the Mrs about the size dilemma. Out of the blue and without hesitation she said "get a 27, it's so much nicer, we can just buy a new desk for the spare room!". So with that in mind I'm going to close this thread, and start a new "sweetspot" thread.
Thanks so much for everybody's input. The 4K was always going to be a compromise, and you were right.😎