use 4k apple tv on non 4K tv?

Yes. Any HDMI device negotiates with the display and outputs a resolution the display can cope with, or you can choose a suitable resolution manually - 60Hz or 50Hz for example. The ATV won't provide a resolution the set can't cope with. Obviously you won't get 4k pictures, but you can watch ordinary HD perfectly well.

As stated it will work, however if you don't plan on getting a 4K set anytime soon it's better to go with the 4th gen as you won't see much, if any difference. Also, 1080i wouldn't be supported so unless your TV supports 1080P it will default to a lower resolution.

I’m running a 4K AppleTV with my 60” 1080p television, and it works just fine. It may just be my imagination, but the picture is just a big crisper than it was with a 1080p streaming box (Amazon FireTV).

We have (I'm a bit embarrassed to say) an ancient 37-inch 720p HD TV and the Apple TV 4K works fine with it.