Ultra HD 4K Blu-rays have the best picture quality possible for your 4K TV. Look out for discs that have high dynamic range (HDR) and wide color gamut (WCG) which will produced a much more noticeable improvement.
Other technical improvement over regular Blu-ray includes:
- 10 bit colour – so more steps of graduation
- Higher frame rates
- H.265 – latest codec
- Greater capacity disc – 4K Blu-rays will have 66GB or 100GB capacity
- Audio – new audio formats such as Dolby Atmos and DTS:X
Source: What is 4K Blu-ray? – CNET