Using one Apple 4K TV for more than one TV

You could, by unplugging and connecting to the other.

You may or may not need to adjust video settings each time, depending on how different these TVs are (resolution, HDR).

I wouldn’t recommend it for user experience, and advise one Apple TV box per TV.

I move my ATV back and forth between two televisions relatively regularly. And, sometimes, I take it to work to use for presentations. The user experience is absolutely fine. And, living alone, where I can only watch one television at a time, I'm certainly not going to buy a second ATV. Having two televisions is probably a bit much but they're harder to move from one floor of the house to the other.

Yes. As long as the television has an HDMI port, you can use an Apple TV with it.