Step 1: Divide the number into separate digits:
9043 => 9 0 4 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 0 => 9
0 to 4 => 4
4 to 3 => 1
3 to 9 => 6
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
9 => C
4 => N
1 => C
6 => N
Step 4: Combine the filters to a string:
C N C N = CNCN