Abstract. For every nonconstant polynomial f ∈ Q[x], let Φ 4,f denote the fourth dynatomic polynomial of f . We determine here the structure of the Galois group and the degrees of the irreducible factors of Φ 4,f for every quadratic polynomial f . As an application we prove new results related to a uniform boundedness conjecture of Morton and Silverman. In particular we show that if f is a quadratic polynomial, then, for more than 39% of all primes p, f does not have a point of period four in Qp.