So...
What is special about the following that doesn't apply to anything else?
4: 4, 8, 12, 16, 20, 24
6: 12, 18
8: 4, 8, 12, 16, 20
9:
10:
12: 4, 6, 8, 16
14:
15:
16: 4, 8, 12
18: 6
20: 4, 8
The non primes down the middle are obvious. Why the multples of 4 and 6 for the rest? (Only 4 and 6 for the non-prime diagonals too...)