I really see no reason not to go for the Apple TV 4k at this point. The ATV is a relatively cheap device anyway and price difference is very minimal between the 4k (170,-) and the 3rd generation (150,-). Besides the fact that you'll probably be enjoying updates for a few more years if you opt for the newer model, the industry have just settled on a set of standards (HDMI 2.1, rec.2020, HDR10, DV, Dolby Atmos) for years to come probably. I don't see those standards advancing anytime soon, because they provide a solid foundation for manufacturers do develop on for certainly the next 5 years, and when manufactures are finally able to take full advantage of them, there is no reason to abandon these soon after, because we're really touching the limits of what our human eyes, ears and brains can distinguish.
Even if you don't own a 4k television set and a Dolby Atmos capable sound system right now. If in the future, you have to replace or upgrade your tv or sound system, you can be rest assured that you can enjoy it to it's full capacity with your Apple TV 4k.