Collecting our results thus far, we are down to the treatment of the primes
PN for which N 15 and N mod 7 = 4. Equivalently, these are the two subcases
N = 4 and N = 11, with which we now conclude. For the P4 case, we may as well
include all prime numbers in the class of digital sum four—this would therefore
overlap with Theorem 2.3 and so to avoid repetition, we will now include the digit
2 in choosing the suitable multiplier.