IMO, 60in sets are about the break point for 4K HD. Anything smaller and I am not sure its worth it compared with current HD formats. Even at 60in, I am not 100% sure its worth it or that others will think its worth it - our friends have a 62in LCD and on SkyHD the HD channels looks fantastic - I don't really know if 4K would be much of a step up even on that size tv.
As an aside note, I know many people with more regular 32 - 42in sets who have NOT bothered getting a HD source (over air or BluRay) as they claimed the improvement wasn't worth it. I can see the same thing happening all over again on these size sets.
As for a 4K source, bandwidth is the issue. It will be a massive headache trying to fit a couple of 4K channels on Freeview, and not much better on Satellite or Cable networks. You can put in online, but how fast a connection would you need, and I don't think at the moment if thousands of people attempted to access the same 4K content at the same time, the internet wouldn't cope - this may change with time, maybe a reason some will once again having a pay for internet option - so that only the pay-for internet route would carry such content. I see that bluRays mastered from 4K are out there, but IMO I cant see this making much difference - its still a current HD picture.
Without 4K material I don't see much, if any reason to pay for one of the new 4K sets. Also, with a 4K set wont upscaled SD look worse than it does now on current HD sets?
And from what I have seen, 8K is already in the pipeline - so unless you have to be on the leading edge of new tech (with very deep pockets) wait the extra year for 4/8K sets to appear.
IMO, OLED screens might be more important than a move to 4K, certainly at the more normal size (and small) screens, where the OLED can remove the current disadvantages of regular LCD (or back lit LED) - eg, improvements other than resolution.