5479

This number is a prime.

The average of the smallest (1009) and largest (9949) 4-digit primes featured in the book "Prime Curios!: The Dictionary of Prime Number Trivia" by Chris K. Caldwell and G. L. Honaker, Jr., 2009. [Green]

The sum of all multidigit composite repdigit numbers less than a thousand, i.e., 22+33+44+55+66+77+88+99+111+222+333+444+555+666+777+888+999. [Loungrides]