Honestly I'm very surprised that my point is falling broadly on deaf ears in this thread, so I'm gonna repeat it because I'm genuinely keen to hear people's reasoning.
Why? Why do we need to go to 4k?
Nothing to do with technology in TVs, bandwidth, or what's available at the studio/source end. I'm saying why do people think we need to go beyond HD and into 4k for broadcast TV? More than half of it is simply not the kind of stuff that benefits from that level of fidelity. It's just not a necessary end goal in my opinion. For clarity I'm not sheltered, I test a lot of the latest TVs before release, I play PC and console games, I watch broadcast TV and stream movies from lots of sources.
Even if we all get 1Gbps fibre connections, the extra bandwidth for 4k is a ****-ton of extra, often wasted, capacity. This is real stuff here, I'm not talking about pixels you won't see without a 77" TV - bandwidth has an energy cost and an environmental impact. Good quality, well-encoded HD will be more than plenty for a huge amount of broadcast content. Remember that broadcast HD right now is something like 5-7Mbps maximum (can be less, see the cheap movie channels). Whereas you can up that to 10-15Mbps and get much better image quality still at HD. There's more left on the table so to speak.
Yes going to 4k is more than the cameras, it's the storage, editing machines, encoders, the entire pipeline.
I'm not saying that current broadcast doesn't need improving in quality, but I'm saying that there is a lot more to be done with HD/current SD first (like cleaning up the vast range of quality on broadcast from fuzzy rubbish right up to reasonable 1080i HD). So again technology aside... Why 4k?