How could you prove that if a^n-3^n is a positive prime number, then a=4 and n is a positive prime number? (all numbers are natural)

Am I supposed to try and factor a^n-b^n to see for which a and b the factorization is possible and for which it's not? I am having trouble with that factorization anyhow, so does anyone know how to approach this problem?

I mean it seems to work for small n, but I just can not figure out how to do it for all natural numbers n...

Thankful for answer!