Kind of a side discussion:
As far as I know, the Apple TV has never "downloaded" movie content, there's no option, or download management, etc., it only buffers up the stream. I don't even know if we have confirmation how much it buffers up - I've seen some people claim right here on MR "the entire movie", but I've never been able to confirm this, even as a developer with those documentation resources available.
Sure, you can disconnect while a movie or show was playing, and it'll run for a limited about of time, again, that's buffering. If you can't stream 4K, does the ATV5 really send the 4K content, and spin for 15+ minutes before starting the movie, based on the bitrate needed and the projected connection speed?
I really wonder if this is documented anywhere. I've lost connection during streaming, but in my experience, there's just a minute or so before it tanks.
I mean, I've tested it this way: started an HD movie from iTunes, pause it, assuming the logic would be to download at the fastest rate possible (vs. the display bitrate), right? I started an HD movie, paused, left it paused for 6 minutes, at a ~6mbps HD bitrate (could be lower or higher by a couple of mbps, just a midrange speed to make the math simple ...), and a ~60mbps connection (no other traffic on the network), that's about 10:1. At that ratio, 6 minutes, 360 seconds@ 60 mbps would be like an hour of download/preload/buffer. At the very least, if it continued to download at the streaming speed 6mbps, you'd get an equivalent real time of 6 minutes.
However, the 32GB ATV 4, with most of the storage available, I can disconnect the network, and it plays back for about 60 seconds. It does the same with a 30 minute TV show like the Simpsons from FXNow.
Of course, this is totally outside of the discussion about downloading it with a device that does, explicitly support such, like a Mac (so that you could stream from iTunes -> Apple TV), iOS devices, etc.