Step 1: Divide the number into separate digits:
9840 => 9 8 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 => 1
8 to 4 => 4
4 to 0 => 4
0 to 9 => 9
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
4 => N
4 => N
9 => C
Step 4: Combine the filters to a string:
C N N C = CNNC