4K doesn't really say anything about image quality, just the resolution of the picture, which tells you the theoretical maximum level of visual detail.
Focusing on resolution is like asking "how strong is one meter of rope" without talking about the composition of the rope.
With streaming video, image quality ultimately comes down to the codec and the bitrate. They probably use a relatively low bitrate regardless of codec.
Bitrate, resolution, and codec are all of course critical, and not knowing all three makes it impossible to judge how good or bad it will look. Sadly the resolution is the only one of the three that's easy to describe to consumers, so here we are.
Focusing on resolution is like asking "how strong is one meter of rope" without talking about the composition of the rope.
With streaming video, image quality ultimately comes down to the codec and the bitrate. They probably use a relatively low bitrate regardless of codec.