Can I use Apple TV Gen 4 outdoors?

I have an Apple TV (Gen 2) and a TV out on the back patio (covered) that's been out there for at least three years. No problems at all.

...that's not to say the the heat/cold, humidity will not shorten the life of those electronic devices. In fact, I expect they would shorten the life, but so far no issues.

think moist and algae and the likes would kill it over time, how much time would depend on the climate you're at

Your TV won't like it much either. There's a lot of space inside a modern TV case, to fill up with... Hate to think about it.