It looks like 4K Blu-ray discs should be a thing by Christmastime.
The Blu-Ray Disc Association, which is an official body that decides Blu-ray specifications, has just a set a standard that will allow ultra HD material to be stored on Blu-ray discs. The BDA took its time because it not only had to increase resolution but also provide better colour reproduction along with the potential for three different types of HDR video.
So, what does all that mean? The new medium for ultra HD Blu-ray will be a 66GB dual-layer disc and a 100GB triple-layer Blu-ray disc, and video will be encoded with h.265 or HEVC (High Efficiency Video Codec). Audio will stick to Dolby TrueHD and DTS HD-MA, which are both lossless and studio-quality.
Previously, Blu-ray used 4:2:0 colour, while the new 4K Blu-ray can mange 4:4:4 (even though most TVs and most AV receivers don't yet support 4:4:4). The new Blu-ray ultra HD specification also added support for 10-bit sampling, an improvement over 8-bit sampling, but likely something you won't really notice.
As for HDR, or High Dynamic Range, it's important because it will allow your TV to show much more detail in dark and bright areas. Ultra HD Blu-ray discs will enable HDR formats. At CES 2015, Netflix and LG announced they would bring HDR via streaming video, so it's now a big deal that Blu-ray can enable the format as well as increase resolution.
And finally, the new specification does not include 4K 3D. We assume 1080p 3D is still in tact, but it's a definite signal to the industry that 3D was indeed a gimmicky trend.