If you haven’t seen any 6-star R3 Champion so far, here is the one you may want to have a look.
The Health and the Attack
Can you Do Rank 4 a 6-star Any Sooner
It doesn’t seem anyone can R4 sooner. It might take a few years I think.
To R4 a 6-star, you will also need Tier 6 Basic and Tier 3 Alpha, and they are not available.