Step 1: Divide the number into separate digits:
0684 => 0 6 8 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 6 => 6
6 to 8 => 2
8 to 4 => 4
4 to 0 => 4
Step 3: Convert distances to codes (0=E[qual],
1 & 9 = C[onsecutive], 2-8 = N[one]):
6 => N
2 => N
4 => N
4 => N
Step 4: Combine the filters to a string:
N N N N = NNNN