And photos
And everyone else's home movies uploaded at 4K to youtube, etc
And 4K Netflix
Etc.
Plenty of applications for 4K besides just MY home movies. Apple has sold more than just one 4K-capable iPhone you know. And there are plenty of non-Apple, non-iPhone people out there able to shoot 4K too. And Netflix has a few shows in 4K. Youtube. High resolution photos that much crisper. What's to stop Podcasts like Hubblecast and similar from embracing 4K if Apple gave them an outlet through which to deliver that 4K to 4K televisions? Etc.
Besides, not $1 can be made on iTunes store 4K videos for

TV until there are 4K-capable

TVs in homes to buy or rent and actually play it. I don't know why so many of us get confused about that. It makes zero sense for the software to already be in the store for sale before there is even 1 unit of hardware to consume it.
How many iOS 10/iPhone 7-dependent apps are in the much more important iOS store right now? Should Apple delay launching the next iPhone until the store is loaded with iOS 10/iPhone 7-dependent apps? Of course not. The hardware must lead the way. If these 4s had 4K built in, every unit sold becomes that more motivating for Studios to test the market for some 4K content in the Store. And as soon as one could make profit on 4K video, the others would rush right in. As is, even if every Studio wanted to roll out everything they have in 4K for

TV tonight, not $1 could be made from all of that "everything" being in the store. Why? The hardware must always lead... not the software.
Apple has 100% control of their hardware. They could roll out an 8K

TV if they wanted to do so. They don't have control of iTunes store partners. By selling millions of units capable of anything new and different, Apple can create the motivation for Studios to want to offer support for anything new and different. Instead, they've left that to their box competitors who are already offering 4K boxes and selling them to those who opt for a 4K-capable box in 2015-16 rather than waiting for Apple to catch up with that feature by 2017 or 18 or 19... or whenever someone internally figures out that THEY want to show their own 4K home movies or photos on their own 4K TV without having to go through a competitor's box... or work through the direct connection route and thus store those big files on their limited-space mobile device so that the video is actually available via direct connection.