4K isn't a fad but it's by no means popular and for the TV sizes that most people can afford for it to be useful. Just like how a retina display works, the 1080p TV is retina at typical viewing distances up until around the 70-80" range. Beyond that it makes sense to get a 4K TV. For the same reason the 5K iMac makes sense as you sit much closer to it. If your couch or chair is pretty close to your TV then a smaller size might make sense. I'm glad my iPhone 6S will have 4K video even though I don't intend to buy a 4K TV for at least a few years. It's always good to have archival footage of my kid that will look better in 20 years on wall sized TVs.