Let R be a ring with an involution * and p, a, q ∈ R. In this paper, we investigate the necessary and sufficient conditions for paq to have a {1, 3}-inverse (respectively, {1, 4}-inverse). In particular, the equivalences for the existence of the MoorePenrose inverse of paq are obtained. As applications, the existence and expression of the Moore-Penrose inverse of a (2, 2, 0) matrix over R are given.