I went to Best Buy and Costco the other day to look at 4K TV's.  Talked to some reps.   The Demo's on 4K you see are 'mostly' 1080p while a few are 4K native.  The Demo was a standardized version identical at BB and Costco.  The rep at Costco pointed out "now your looking at 1080p on a 4K TV" and now you looking at 4K in this segment of the demo. 
 
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4K is like 3D, not available for FREE
Hard to find 4K content is MUCH worst than finding 3D content
4K is a joke

3D  (2011) is still evolving and next models . . .
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They've all ready fixed the 30 FPS cap with display port in the newer models. 4K content is still new and in the early stages. I remember having a 1920x1200 resolution screen when 720p content was mainstream. It's really all about how you plan on using the display. If you want it for watching stuff on cable TV there is no reason to, because they don't offer 4K content yet... Just like they did when people bought a 1080p TV when 720p content was mainstream "HD." Also, we are running into a wall at this point, since 4K content takes up so much data... You have to have a very fast internet connection just to buffer it. I'd love a 4K TV for gaming purposes, especially in FPS games where I would highly benefit from the larger field of view.