Wanted to add: 4K and HDR support among streaming services has gotten a lot better recently, especially now that iTunes supports it via the new Apple TV. If you've been waiting for 4K to take off before upgrading, now might be the time.
Netflix, Amazon, iTunes, Google Now, and VUDU all have an increasing amount of 4K and 4K+HDR content. LG's 4K TVs have great built-in apps for all of those services except iTunes.
Quality-wise, I don't think 4K is nearly as noticeable an improvement over 1080p HD as HD was over standard-def. It looks great, but from five or six feet away most people wouldn't be able to tell the difference.
What really does make a difference, though, is HDR, and especially Dolby Vision HDR. It allows a wider color gamut and much broader contrast range, and the result can be absolutely amazing, especially for colorful movies (like Guardians of the Galaxy Vol. 2 and The LEGO Batman Movie) or for things that have lots of dark/light contrast. Documentaries like Planet Earth II will absolutely blow you away with how real they look.