I agree with davidjschenk.
As big as 85" is for any HDTV whether it be 4K or less, it "may" be difficult to fully appreciate 4K's higher resolution over 1080p even at that size unless you sit pretty close to the set which would likely be uncomfortable and/or impractical in most cases.
I'd say an 85" HDTV is where you "may" begin to see a little bit of the benefit of 4K resolution if any but really, you'll likely need 95" + of screen to truly see a difference between an excellent native 1080p source and a native 4K source.
In spite of the all of that, I know it's very difficult not to be highly impressed with a lot of the 4K TVs demoed at Best Buy or elsewhere with their hyper bright, detailed and crystal clear images. Of course they're displaying 4K content that was shot solely for the purpose of showing these TVs at their best and that certainly shows in a lot of what I've seen on sales floors. In fact, it serves it's purpose and makes me really want one in my home. Great marketing! It was the same way when good 1080p sets first hit the market and were shown off displaying high quality 1080p source material except that 1080p is very clearly far superior to 480i at screen sizes above 42" inches or so. The difference between 4K and 2K isn't nearly that obvious unless the screen is HUGE.
If however, you were able to put a 65" 1080p HDTV side by side with a 65" 4K (UHD) TV and display the same exact (shot in 4K) content on both while they were in comparable picture settings/modes and in the same viewing environment, you'd likely have a very hard time seeing any difference in resolution on the 4K set as long as the TVs are of similar quality.
On top of that, at this time the only benefit to 4K is the higher resolution. There is no other benefit over 1080p HDTV other than the higher pixel count which necessitates a much larger screen than even 65" to fully appreciate. There's no benefit to black level, native contrast, color reproduction or any other parameter of picture quality.
Future proofing if possible is of course a great idea. A few of the 4K sets I've seen look fantastic in the store with the 4K material used for demo purposes.
Buying a 4K set isn't really the issue, seeing the benefit of that extra resolution on any screen sizes below 95" is.
Be sure to do your homework on the sets you're considering and try to choose one that has the best possible upscaler. Also, one that rates highly in all other aspects of picture quality such as native contrast, black level performance, motion handling or any other performance parameters that are important to you, your viewing environment and how you watch TV.
Anyway sir, best of luck with your decision and your final choice.
