With offering 4K and Sling TV, you have to start somewhere, sometime. This is a start. Just like with HDTV it had a start, along with tv altogether.
With 4K Joey, it receives content from the Hopper's tuner. I assume the 4K Joey would be decoding the 4K signal which would be cheaper to do than making a new Hopper 4K. Most likely the living room has the Hopper and that would be the room that has would need 4K. It would have to be swapped with another room to have the 4K Joey placed there.
What would be nice is a solution to stream content to the digital channels on all the tv's in your house or at least be able to do something similar to what the dual tv out receivers were able to do (322,522,722). It could receive the content from the Hopper without a box required in the rest of the rooms transmitted to a certain digital channel on the other tv's in the house. My guess is that the cost is too high to do this at the current time.