I think there's a lot of suppressed envy out there when it comes to this topic. I remember reading it when we started seeing 720p vs 1080p debates. I saw the Sony 4K TV in the Sony Style store in San Diego back in 10/2012 and several times since. Unless you have bad eyes or sitting VERY far away, there is a difference. My eyes are not 20/20 and I could see a difference and not just sitting 5 feet from it. Some of it is picture quality - no doubt. The other part is truly clarity. I could see details that aren't there on my Pioneer Kuro especially when you add geometric shapes such as bricks, side-walks, etc. But you also see it in beaches, close up of people, and other material. Another area I noticed it was straight edges; at 1080p some material will be blurred or aliased. The material demoed at the store had a beach area, the depth of field was much greater; it was easy to see wave peaks at distance where my TV would make it look like white foam or just ripples.
I'm an amatuer photographer, but I have been shooting digital SLR since the Canon D30 - consider the first mass market Digital SLR. What improves between resolution, with all things being equal (f-stop, shutter rate, image processor, etc) is the depth of field resolution and clarity up close. If you compare a 4 Megapixel F8.0 image and a 12 megapixel F8.0 image, the 12 megapixel will be sharper. Is it possible for a 4 megapixel to look better, sure source material, processing, color depth, quality of lenses, etc play a big part... but all things being equal there's a difference; and the same can be said for a 4K TV.
Is it possible to sit far enough back that there's literally no difference between a 1080p and 4K... absolutely. IMO, I don't think it is practical on TV's smaller than 65" (assuming normal viewing distance) but I think at 65" and above, you will see a difference and at 70" to 80" absolutely.
Ask yourself this, could you see a difference on the current 65" playing 720p vs 1080p. If the answer is yes - why wouldn't you see a difference when going to 65"+ with more resolution?
If the price was say $7K, I would consider buying one today; probably only buy the Samsung since it's suppose to be upgradeable. At the current prices, I would rather add a room to my house and make a dedicatd theater.