4k is for editing for now (maybe even long-term). The reason is that outside a few high-end home theatre setups, 4K is simply not necessary from the viewing point of view. You've got to get over 60" display size and be relatively close for 4K to make much of a difference. The main reason to buy a 4K display right now is for non-resolution related advantages.
Then there is the content problem. Hardly anything is 4K, and unless the telco monopolies are broken, we'll not have good enough internet to really stream 4K anyway (heck, we can't even stream good 1080p yet... not even CLOSE).
I'm sure it will happen one day. *Maybe* we'll move to over-60" being typical for displays, but as homes get smaller, I'm kind of wondering about that one. And, as more consumers buy 4K, I suppose there will be pressure to make everything 4K, just because. But, Apple probably isn't going to be an early adopter of that kind of move. They usually don't do the 'just because' type fads (or, at least they traditionally haven't).