Step 1: Divide the number into separate digits:
4313 => 4 3 1 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 3 => 1
3 to 1 => 2
1 to 3 => 2
3 to 4 => 1
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
1 => C
2 => N
2 => N
1 => C
Step 4: Combine the filters to a string:
C N N C = CNNC