I think it's the next major update to blu-ray spec that we can expect. In my opinion, more important than 4K support is full adoption of Rec. 2020 specification, which defines UHDTV, and upgrades video encoding to 10-bit (and 12-bit) with a new colorspace closer to that of digital cinema, encoding in both AVC and HEVC, and a finer gamma correction (or even a linear transfer function, similar to raw data). To accomplish this, the most important thing to assure quality is bandwith. Digital Cinema today is still 2K, which is almost FullHD, and images are stellar, fill big screens, yet nobody complains. I don't think we need 4K right now at home. Color resolution is more important. Nowadays blu-ray is 8-bit 4:2:0 YCbCr encoded in 16-235 luma range and 16-240 chroma, which is less gradation than sRGB JPEG files allow. We desperately need 10-bit (deep color) at least to overcome this limitation.
The problem is backwards compatibility. We would need to upgrade our blu-ray players to support the new format and this takes years. With 4k mastered blu-rays there is margin to improve quality, and recent players do internal high-bit processing and improve the signal to reduce banding. With good compression, careful grading and enough bandwidth images still look great in 8-bit.