Constacyclic codes form a well-known class of linear codes, and are generalizations of cyclic and negacyclic codes. In this paper, we determine generator polynomials of all constacyclic codes of length 4 ź m p n over the finite field F q with q elements, where p , ź are distinct odd primes, q is a power of p and m , n are positive integers. We also determine their dual codes, and list all self-dual constacyclic codes of length 4 ź m p n over F q .