Don't Wait if You Don't have a 4K TV
If you don't have a 4K TV and don't plan to upgrade in the next year or two, don't wait for the new Apple TV to buy. This advice also stands if you plan to buy a cheap 4K TV that doesn't include HDR support.
The biggest new features will focus on 4K and HDR support, both of which you will need a recent 4K TV to enjoy. If you plan to plug this into a 1080P TV that you are going to keep for at least a year, go ahead and buy the current Apple TV.
You will be able to use the 4K Apple TV with a HDTV, but you won't see all the benefits, so you can plan ahead, but that's only worth doing if you will upgrade your TV before you buy another Apple TV and if you plan to buy, rent or stream 4K content which also comes with higher costs.