I think that there is a better case for not including 4k on the ATV4, than Apple not including 1080p on the ATV2. For one thing, the ATV1 had 1080p.
Not true, The 1 maxed with a limited version of 720p unless someone physically hacked it with the Broadcom card. I considered the "1's" version of 720p the bare minimum to allow marketing to claim it was HD. I can't remember but I think that was 720p/24fps or 720p/30fps but definitely leaving out the other of those 2 and 720p/60fps too.
And don't get me wrong, I loved that version myself but it was definitely bare minimum "HD" and definitely not capable of playing 1080p content without a third party hardware hack.
Also there was a lot more 1080p content in 2010, than 4k content in 2015.
In the iTunes store or generally? If the latter, I'd associate that with the broader availability of Blu Ray discs because Discs were still THE primary way to consume video content at home. But here's the reality of all arguments about source content- pretty much EVERY film available at 1080p is readily available at 4K, not because of how many discs are out there but because everything shot on film can pretty much have a 4K render pretty quickly. So, why isn't so much more available in 4K now? Because there isn't a huge number of 4K players in homes yet.
Instead of seeing the lack of content as rationale against a 4K

TV, try seeing it as an opportunity. Reminds me of this shoe salesman story...
2 shoe salesman were sent to into the deepest jungles of Africa in search of new sales opportunities.
- One replies back, "Unbelievable opportunity here- nobody has any shoes."
- The other replies back, "No opportunity here- nobody wears shoes."
The rumor is that a 4K

TV is coming:
- "Unbelievable opportunity- most people do not have a 4K player in their homes yet"
- "No opportunity here- there's almost no 4K content to play" OR
- "No opportunity here- almost nobody has a 4K TV" OR
- "No opportunity here- I (er I mean) nobody wants to replace a perfect good TV set" OR
- "No opportunity here- I (er I mean) nobody can see the difference" OR
- "No opportunity here- broadband everywhere is not upgraded to handle it" OR
- "No opportunity here- need more storage for bigger files" ETC.
With 4K, it is not a chicken or egg proposition. The hardware MUST lead. Put lots of 4K-capable hardware in homes and some Studio(s) will be tempted to see if they can make a buck on 4K versions of their film libraries. If they do, all of the other Studios quickly follow. Boom- lots of 4K available in the iTunes store.
This doesn't work the other way. Wave the magic wand and make EVERYTHING in the iTunes store have a 4K version for a 4K

TV. How much money can be made on all of that? ZERO. Why? Because there's no 4K

TVs in homes to consume that software.
The hardware must lead the way. And Apple completely controls the decision to create & launch hardware capable of playing 4K video. They don't have to wait for anyone else to do anything.
Good example: look at the apps in the

TV app store. Before the

TV4, there were hardly any third party apps because there was no store through which to offer them. Then, Apple rolls out a 4 with a channel to offer third party apps and now there are LOTS of apps that were not available in the "3" and before. Guess what happens if Apple rolls out a "5" even if at first there is almost no 4K content in the iTunes store? The very same motivator that moved the various "4" app creators to create software for the "4" will motivate the owners of lots of content that could be 4K to make it available for the "5".
The 4k TV adoption in the US was much lower, < 1% in 2015, while the 1080p TV adoption was much higher in 2010.
First, Apple rolling out a 4K

TV doesn't require a 4K television to use it. Better hardware can feed 1080p, 720p or SD to <4K sets. There seems to be a very strong perception (or spin?) that a 4K

TV REQUIRES a 4K TV and that's not true. Did a 1080p

TV require a 1080p HDTV? No. This will be no different.
Second, where are such arguments offered
against Apple for putting 4K video capture in iPhone or making 4K editing capabilities in iMovie & FCPx, or supporting 4K video in a Quicktime container that stores just fine in iTunes. Wasn't it stupid of Apple to make iPad pro market the capabilities of editing "up to three 4K streams"? And on and on. In other words, arguments like this one seem to support why this ONE product from Apple should not be upgraded, yet you won't see this same kind of rationale argued against 4K capabilities being put in pretty much everything else Apple offers. All the other products have 4K and Apple is not stupid for building it in there. However, this ONE product does not yet offer 4K and since Apple had not yet endorsed 4K here, nobody needs a 4K

TV.
Third, when does Apple wait until all of the other players have everything in place before Apple can advance it's own hardware? Did printers, USB sticks, external drives, etc all have Thunderbolt in place before Apple adopted Thunderbolt? USB3C? Lightning? Does everything that plays audio have Bluetooth and readily & immediately connect to AirPods? Does all Mac software already support the Touchbar? Do all retailers support Applepay now years after Apple implemented that? And on and on.
For some reason, with this ONE product from Apple, we believe (or spin?) that before Apple advances this hardware, the other players have to do a bunch of stuff first... or homes have to be full of 4K TVs before Apple should make a 4K

TV. However, look at everything else that Apple makes and they roll out all kinds of hardware advancements that are still waiting on everybody else to "catch up."