When’s a good time to buy into 4K? Now. With caveats…
HDR – make sure it supports the following: Dolby Vision, HDR10, HDR10+ and HLG)
nits – you want 1000+. Anything 350+ will be an improvement over SDR.
Input Lag – anything below 30ms is great. This is now achievable in a number of 2017 models (and some 2016). Pay attention to what features are available while in game mode–sometimes they are more restrictive.
Verdict: buy now with adequate research
I’d wait for something that fully supports VP9 profile 2, which is what is required to view HDR on YouTube. At the time of writing, only two external devices qualify (Chromecast Ultra and the Xiaomi Mi Box). There are a number of televisions that support it, but their metadata collection is typically a showstopper for me.
I also want the ability to do Atmos passthrough / DD+.
The Nvidia Shield would be my overall pick, but it doesn’t do VP9 profile 2. Maybe there will be a refresh down the line.