I think it would be helpful to know what your intentions with the ATV are. If you are just going to use the same apps that are on your ATV2, using the TV's built-in apps should suffice.
I think the ATV4K is the best streaming box out there, but most of the streaming apps are very similar between the ATV, other streaming boxes, and built-in apps.
If you plan on using stuff like iCloud Photos, Apple's content, or games, the ATV4K would probably be worth it.
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If you end up getting an ATV, get the ATV4K. It is much better than the ATV4, but only a little bit more $ if you are paying retail prices.
Also, the 32GB ATV4K is often on sale at many different places. It has ended for now, but every once in a while DirecTV Now has a deal for a free on if you prepay for 3 months of service, which still ends up being much cheaper than full price.
I bought the ATV4 at launch, and I think that it is underpowered. It launched with a A8 chip that was already a few generations old at launch, and is now much older. The ATV4K at launch had a current A10X chip.
I think the price difference between them is $20, and imo, a great deal. Or maybe the ATV4 is just a horrible deal.