Although Brunn was the first to make a 4 mirror design the Stevick-Paul is more widely known. Stevick-Paul telescopes are off-axis versions of Paul 3-mirror systems with an added flat diagonal mirror but to me it’s just a Schief behind a Schief. It is well corrected although it has a lot of field tilt. With 4 mirrors it will require enhanced coatings of the light loss will be considerable.
Optical Ed's 