One thing about 4K TVs is that the top of the line TVs are 4K, so in most cases you will get better picture quality and features with a 4K TV. I have a 4K 40" Samsung HU7000
(American version review) but it's in my den with my computer and sits between 40" and 48" away. The picture quality is great. I don't know if it's because it's 4K or it's just a newer and better TV than my previous 3D Sony 40" TV. It came at a very good price so I decided to check it out.
This argument reminds me of the 720p versus 1080p argument a few years ago. There aren't many 4K sources right now except a few on Youtube and on Netflix but that will change and these TVs do a good job of upconverting.
Here's another graph which makes my decision reasonable as my viewing distance is right in the sweet spot.