Ever since the iMac 5k came out i looked for this same answer. When doing my reserch i found out in a fourum, that to edit 4k you need an 8-core processor, and 4gb video card, but since the iMac had a 5k display it would need at least a 5gb video card which is not an option anywhere right now. Since then i'm still waiting on the new iMacs and Macs Pro, but if they dont pull a good system, im seriusly thinking about jumping back to windows.
In the mean time im using proxy files, and optimized media and im loving it! Makes my old iMac feel new and fast.
There are many many articles of speculation about what Apple's future plans are, so I won't go into extra detail here, but as a person that has been quite close to the Apple ecosystem since 1987, I have to say I'm not hopeful.
Apple has been dragging it's feet about true professional systems for a while now, and has either abandoned (ie Aperture) or dumbed-down (ie Final Cut) many of the products on which they made their
professional reputation. They have done this with hardware too. In Apple terms, sales of more capable products like the Mac Pro are slow compared to other products that account for 2/3 of their income, like the iPhone.
Not so long ago, Apple Computer Inc. became simply Apple Inc. What you might read in this change is that Apple's scope is much broader now, into all
consumer devices, as they look for another runaway hit like the iPhone that singlehandedly made them the most valuable company in the world. And it's not necessarily a computer. It could be a car, or maybe something else with broader appeal, sooner ie a toaster? Blender? HomeKit driven Washer/Dryer? They are trying to make the next
thing that every person on the planet both needs and wants. As much as we wish it was so, that thing is not going to be a high horsepower video editing machine.
In terms of computers, Apples improvements over the last 6-7 years have been really only incremental. Screen resolution bumps, processors, etc. But nothing huge -- my 2010 MacBookPro laptop is 90% as good and useful as the best MacBookPro sold today. Meanwhile, iMacs get faster cards and video, but none have the architecture to handle a 5K file, or even a 4k file that well. There are many bottlenecks there.
People doing pro 4k and 5k are
making do with MacPro "trash cans", but still have to depend on
large peripherals to get the job done. The irony here is that Apple once made the perfect box for these kind of users -- the
2006 - 2013 Mac Pro. Lovely machine. I'd go back to one if I could just get the video and Thunderbolt2 cards needed for modern workflow. And the new MacPro hasn't seen much new in development either. Everyone I know in high end production is ready to jump to a new platform* if they can get the technology bumps they need. They all love the MacOS, but it has become an increasingly difficult thing to stay with it when Apple seems deaf to their needs.
We are .001%. Apple is designing for the largest audience it can. The next device that everyone has to have. And I doubt that thing is a desktop computer.
*by "new" platform, I don't mean Windows. There's a lot of interest in a platform that is "open" like a flavor of UNIX, but that is specifically tailored to media production.