Actually, I force my ATV 4K into believing it's connected to a 4K SDR TV by using a HD Fury Linker, which also downscales the 4K to 1080p for my Panasonic plasma.
The benefit of doing this is that you then achieve the much higher 4k streaming bitrate over the HD bitrate, which results, imo (and my wife's), a better picture. It's not quite HD Blu-ray quality and the sound quality takes a hit over HD Blu-ray, but as mentioned above, if you're going to change your TV to a 4K one eventually you'll be able to watch all those purchases in HDR.
I had the idea a few years ago after reading up on Profile 5/LLDV and coming to the conclusion that a device implementing this should be able to perform HDR to SDR conversion correctly (there's not many non-profile 5 devices that can achieve this correctly, or at least, there wasn't at the time). The downside is that you have the extra HDMI handshaking between the ATV/Linker and Linker to (in my case) AV amp; which adds a few more seconds to the handshaking.
