I have a 2014 model 4k (Samsung HU8550) and a 2015 model 4k (Samsung JS9000). I'm not unhappy with the 2014 model, but I wouldn't recommend it. The 2015 model, however, is a quantum leap over any TV I've had. It's not the 4k resolution that makes the difference, IMO, but rather (1) the HDR (= High Dynamic Range) which gives you bright highlights, (2) WCG (= Wide Color Gamut), which gives 92% of the color gamut that is standard in cinema, and (3) upconverting 1080 resolution to 4k, which needs some computational capacity to do well.
But you have to buy a 4k set to get the other, probably more significant, benefits, available on premium 2015 models. It's expensive to get a premium model, though. I doubt that an economy 4k set is a good deal, now.
