43

This number is a prime.

The largest prime less than a thousand which remains prime when 4 is added to any of its digits, i.e., 47, 83. Note that the concatenation of these new primes, 47, 83, i.e., 4783 is also prime. Furthermore, the concatenation in order of the first prime, 43, and the new primes, 47, 83 is also prime, i.e., 434783. [Loungrides]