Sony had an answer for that right now. It comes in their 4K Ultra HD Media Player (FMP-X1) which is, for all practical purposes, a specialized hard drive with enough "oomph" in electronics and space to hold and parcel out 4K video — that takes a lot of "bandwidth" as you can imagine. Sony's advantage comes from their having a movie studio that has been producing 4K-resolution movies so that there are films ready to roll right away. Plus Sony has the smarts to know how to take films and turn them into 4K-resolution video for home viewing.
The 4K Ultra HD Media Player has a trick up its sleeve even before you get to looking for a 4K film to watch. Connected to a Sony 4K Ultra HD TV, it automatically brings up a menu that otherwise is never there. This connection, by the way, is a simple one that the homeowner can do: one cable goes to the wall outlet for power and the other into the TV and that's all there is to it. Now the menu shows the movies that are available to rent for one full day (typically at $7.99) or own (typically at $29.99). Titles include a wide assortment of films and TV shows that range from "The Amazing Spider-Man" to "Breaking Bad." Of course the number is still small, but it's growing rapidly and there's sure to be something worth watching in 4K quality. Accessing all of this is along by the 4K video service that Sony has built up and built in for home owners. And which makes the term "home theater" a lot more powerful than it ever was.
But it's the way the Media Player gets the films that really counts. It uses the Internet to download the entire movie (either with a wired or wireless connection), rather than trying to stream it "as it goes." Even those with fast connections know that it takes more than just a few minutes to get a movie streaming (in HD). And any person who's gathered the family around to watch a film can relate to the horror stories about glitches, server errors and other problems that happen when streaming — one of the worst being trying to watch something popular on a Saturday night or holiday when too many others had the same idea. So downloading the movie first means removing most of the problems, although there's going to be a wait. But all that means is a bit of thinking ahead; something that anyone who knows and appreciate the benefits of 4K has already planned for.
Sony's Media Player is only compatible with their 4K Ultra HD TVs, so it won't do any good to try and connect it to a different manufacturer's model. In this way Sony is able to maintain a level of quality and avoid any compatibility problems between it and the display. Plus it's price has decreased to make it more affordable (and can even be acquired for free). So there's no reason to keep asking where the 4K content is. It's here.