Back in January, the HDMI Forum unveiled its new specifications for the HDMI connector, called HDMI 2.1. Now, that HDMI specification is available to all vendors who have already adopted HDMI 2.0. It's backwards compatible with all previous HDMI specifications.
The focus of HDMI 2.1 is on higher video bandwidth; it supports 48 Gbps with a new backwards-compatible ultra high speed HDMI cable. It also supports faster refresh rates for high video resolution -- 60 Hz for 8K and 120 Hz for 4K. The standard also supports Dynamic HDR and resolutions up to 10K for commercial and specialty use.
This new version of the HDMI specification also introduces an enhanced refresh rate that gamers will appreciate. VRR, or Variable Refresh Rate, reduces, or in some cases eliminates, lag for smoother gameplay, while Quick Frame Transport (QFT) reduces latency. Quick Media Switching, or QMS, reduces the amount of blank-screen wait time while switching media. HDMI 2.1 also includes Auto Low Latency Mode (ALLM), which automatically sets the ideal latency for the smoothest viewing experience.
You know how it's already something of a pain in the butt to figure out whether the 4K display you're looking at actually support all the bells and whistles that make 4K "worthwhile," like HDR? Well, actually, if you're reading this then you probably don't, but the 4K label is something of a mess right now, with multiple competing 4K flavours, most of which aren't worth wasting time or money on. Well, you can add VRR, QFT, QMS, and ALLM to your shopping list of "must have" UHD-HDR-4K features.

Confused yet? Don't worry, engadget have this handy chart to help you figure it all out:

See? Simple as cake.

Sarcasm aside, this really serves to reinforce why I don't think 4K is going to become a thing. Not only is there no 4K broadcast standard yet, and no legislative push happening (a la HDTV) to mandate one, but 4K itself has already evolved itself right into obsolescence before most consumers even knew it existed. Seriously, will you still shell out $1400 for a 4K monitor, knowing that any 4K display that you look at today will be missing all the features that you'll absolutely want five minutes from now? Unless you're a hardcore early-adopter, the answer is probably, "No."

And all that assumes that you were interested in 4K in the first place, which most consumers aren't, but... well, I've covered that already, haven't I?

No, I stand by my earlier assessment: 4K is not, and will not become, a thing. Ever. Save your money.