Since we just bought a new house that has a defined theater area (and not just a separate den), we *JUST* bought the 2016 model Sony 75X940D. For what 4K content we've watched on it (Oppo 203 UHD player - Planet Earth II and Passengers, YouTube, Netflix and Amazon), 4K was worth the price, to me. We have ATT Uverse , so no 4K content will be delivered over that ...ever, Grr... (and I'm *NOT* changing to DirecTV). But, the upscaled HD content looks better than I thought it would. We've got the 850a up in the living room now, and I'm still completely happy with it on HD content (I actually prefer it to the upscaled content on the 940D.)
So, IMHO, if you don't have much of an option for 4K content (no Netflix/Amazon or UHD BR player), I'd say hold on to your money. In my case, we needed a larger screen for the theater and made the jump to 4K (which I was more than happy to do when the wife said yes, of course =D ). The 850a does a fantastic job with HD content and still looks amazing, so it is still the main viewing TV in the house.