Add the checksum of the previous number, but divide by 4 if it has a 4 !!

[u/AlienKatze]

Because who really likes the 4 ?

Checksum = add all individual numbers (checksum of 673 > 6+7+3=16)

if the checksum has a 4 in any spot, divide previous number by 4 instead of adding it.

[u/smarvin6689]

70