I bought the ATV 4K 2017 version two years ago when I still didn’t have a 4K TV, but I wanted to be ready when and if I ever did. I did buy a 4K TV a year later, so now I’m glad I bought the latest technology rather than saving a few dollars to get yesterday’s technology. I would have to verify, but I suspect the chips in the 4K version I bought were newer and better than those in the HD version. The ATV 4K 2021 version definitely has better chips.
With nearly every internet provider providing more than fast enough service these days, I wouldn’t worry about upgrading to a 4K TV someday. I do suggest that if you do eventually upgrade to 4K TV that you get an HDR capable one. 4K adds more resolution which you might notice or not depending on how close you are to the set, but the really big improvement is in the extra colors and greater dynamic range provided by HDR. HDR takes you from the 16 million colors from non-HDR to a whopping 1 billion colors, so color banding virtually disappears. And blacks look blacker, not just dark grey. Unfortunately, HDR content isn’t widespread yet, but it will be someday.