Is a 4 color system (white, green,.brown, black) really any more "old-school" than any other rank system with colored belts? When Kano introduced the belt rank system, it was just white and black. I've seen suggestions that Mikonosuke Kawaishi introduced the idea of other colors for kyu ranks when he was teaching in Europe, but I haven't been able to find definitive info on what those colors were. I've found one source saying they were white, yellow, green, blue, brown, black and another saying they were white, yellow, orange, green, blue, purple, brown, black. Neither source provided supportive documentation, so I don't know if either is correct.
I don't think I've trained in a system that uses white, green, brown, black. I've encountered: white, green, black (Bujinkan); White, blue, purple, grown, black (BJJ); White, green, blue, brown, black (YDS), no belt rank system at all (Muay Thai, WT, multiple others. I'm just starting in Capoeira, which has a cord system which varies by organization, but typically is built around the colors of the Brazilian flag (green, blue, yellow, and white) in different combinations.
I don't think I've trained in a system that uses white, green, brown, black. I've encountered: white, green, black (Bujinkan); White, blue, purple, grown, black (BJJ); White, green, blue, brown, black (YDS), no belt rank system at all (Muay Thai, WT, multiple others. I'm just starting in Capoeira, which has a cord system which varies by organization, but typically is built around the colors of the Brazilian flag (green, blue, yellow, and white) in different combinations.