Step 1: Divide the number into separate digits:
4259 => 4 2 5 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 => 2
2 to 5 => 3
5 to 9 => 4
9 to 4 => 5
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
2 => N
3 => N
4 => N
5 => N
Step 4: Combine the filters to a string:
N N N N = NNNN