On the other hand, a plain old Angulon would need a center filter more than a Super Angulon would, even if both are wide angle lenses.
Cos^4 is the falloff expected from a simple symmetrical lens with no "fancy tricks" - a perfect pinhole would give cos^4 fall-off. By using a large strong negative element to "tilt the pupil", also known as Biogon-type (even if the Biogon wasn't the first), the fall-off will approach cos^3. Most lenses come nowhere close to this ideal, and the difference really only gets significant when you get around 45 degrees off the lens axis - meaning about 90 degrees coverage. The wider the angle, the larger the difference. At 100 deg angle of view, a cos^3 lens loses about two stops, a cos^4 lens almost three.