@habib mistake done by you is,
there are 4 possibilities of forming a chain of length 3.
1,2,3 in same slot, 4 in diff slot,
1,3,4 in same slot 2 in diff slot
2,3,4 in same slot, 1 in diff slot
1,2,4 in same slot 3 in diff slot.
it means 4C1 * 8C1 * (1/8)^3*(7/8) shud be the possibility of forming chain of length 3