9967

This number is a prime.

The largest 4-digit left-truncatable prime (emirp). It is also the largest four-digit deletable emirp. Deleting one digit at a time, we take a prime at each step, i.e., 967, 97, 7. Note that 967, 97 are also emirps. [Loungrides]

The largest 4-digit prime number whose sum of digits is prime. [JoJo]