So you agree, in answer to the question posed: How was this done?
The answer can be contributed to compromises in shadow quality. Which is what I was getting at.
You said Uncharted 4 has very poor shadows, and implied that's how it achieves amazing visuals. Uncharted 4 firstly doesn't have very poor shadows, it has mostly good to very good shadows, but with the odd issues here and there, but those compromises are likely a very small part of why or how Uncharted 4 manages to be as graphically accomplished as it is.
I think it likely has far more to do with optimisations, tools, art priority, developer talent etc, as it does compromises to things like shadow quality or whatever else, especially since there are so few compromises compared to other console games.