Knute Reindahl (1857-1936), the Norwegian/American luthier employed the 4-piece purfling method in some of his early instruments, having learned it from his Danish teachers, Peder Adamsen & Adamsen's student Peter Paulsen, in Chicago. Adamsen had learned it from his teacher, Frederick Hansen in Denmark. There were also a number of other Danish makers during that period that used the 4-piece method. It was faster and simpler than the 3-piece method: In order to have a wider center 'strip', they shaved both dark & light strips the same thickness, thereby not having to change thickness settings on the plane; then cut the light strip into two pieces and merely doubled them to obtain the thicker light center 'strip'. I don't know that this method originated in Denmark, however it was used there in the very early part of the 19th century. Jens Stenz, maker in Denmark, is the expert on Danish violin making, and has written extensively on the subject, including their use of the 4-piece purfling method.