Hi Rosemary & Micah,
I have an easier suggestion that worked with my iPad when I wanted to update the OS which you can only update over wifi. Use you iPhone’s hotspot feature. Then ATV or other device will think you are connected to Wifi. As to the SPEED to stream 4K movies, That is the question. But most hotels have pretty slow wifi anyway. So there might not be much difference. I realize that some people don’t have unlimited bandwidth. I do pay for unlimited bandwidth. So maybe this will not work for some people.
I use that trick at work to update my iPhone SE (company phone) and iPhone 13 Pro (private phone). I switch the hotspot on one on and update the other, then reverse. Works very well.