With both a lack of players and a lack of disks, there’s no real reason to jump into the 4KBD fray just yet. However, Ultra HD Blu-ray will, for the most part, look better than streaming and Blu-ray. How much better they look is going to depend on the original movie’s picture quality and medium, and how well it was transferred (or remastered) to 4KBD. This was true of Blu-ray and DVD as well.
Will there be 4KBDs that look no better than regular BD? Probably. Will there be some that look no better than streaming? Doubtful.
Interesting discussion of whether 4K Blu-Rays will be any better than current media quality. Executive summary: a lot of 4K Blu-Rays will be upsampled from lower resolutions.
This is quite a messy technological change. I will quote something from one of the comments that highlights how complex all this is:
One fact that was left out but important is the evolution of HDMI to v2.0a and HDCP v2.2. This is the new requirement for native UHD 4K sources and its not forward compatible with older versions of HDMI, meaning you cannot plug a new UHD 4K Blu-ray player into your AVR or PrePro unless the player offers dual HDMI outputs, one for the new video resolution and one that still meets v1.3 or v1.4 HDMI spec. Be sure you have the new HDMI v2.0a, its the only one with HDR, Dolby Atmos, and true native 4K pass through.
Your average TV viewer won’t understand any of what is in that paragraph above. TVs have become more complex than computers, and this is a Very Bad Thing for the TV industry, and for all of us who own TVs. The number of technologies that need to be compatible is much more complex than those needed to transfer files from Windows to a Mac. (Remember how hard that used to be?) 3D TV is pretty much a failure, probably, in part, because it’s too complicated (and the stupid glasses). I can’t see these new technologies hitting critical mass for quite some time, as long as it’s so complex to make sure that all your hardware (especially if you have a home theater system or AV amplifier) is compatible. And, of course, one of the main reasons for all these complications is DRM…