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.
TV tonight, how much money could be made by the content owners? NONE. Why? Because there are NO 4K