4-2-1 is generally 2 piece, while 4-1 is usually 1 piece, correct me if im wrong?? the 4-2-1 is for those of us who drive the sohc civics, it helps with low end power... the 4-1 is better for dohc civics for the same reason there... basically for different applications. i myself am sporting a 2-piece dc replica 4-2-1 SS header.