400

This number is a composite.

400 is the smallest number m such that |m^k -prime(m)| for k = 1,2,...,5 are primes. [Firoozbakht]

(400/004)! + prime(400) is prime. [Firoozbakht]

400 is the only known square of the form 1+p+p^2+p^3, where p is prime or composite. (Note that p=1 yields a square, but 1 is neither prime nor composite.) [Noe]