The fundamental form of almost-quaternionic Hermitian manifolds

We prove that if the fundamental 4-form ΩΩ of an almost-quaternionic Hermitian manifold (M,Q,g)(M,Q,g) of dimension 4n≥84n≥8 satisfies the conformal-Killing equation, then (M,Q,g)(M,Q,g) is quaternionic-Kähler.