First, make sure you are actually looking at 4K rides. you can check this in settings.
If you are, that's great. We do everything we can to make the videos look as good as possible, but there are limitations.
FulGaz rides are shot on GoPros on bikes, and efficient compression becomes difficult when the subject is constantly moving. This means they can never look as good for the same file size as slow moving objects filmed with a cinema camera and lenses.
FulGaz 4K rides have a bitrate of around 21Mbit/s. We are looking at ways of providing a significantly higher bitrate but this presents issues that need to be solved first, namely the sheer size of the files makes them a download only option for 99% of our customers, and it would be difficult to fit more than 2 hours of content on even the biggest Apple TV.