Equivalence between the Osserman condition and the Raki\'c duality   principle in dimension four
  We show that 4-dimensional Riemannian manifolds which satisfy the Raki\'c duality principle are Osserman (i.e. the eigenvalues of the Jacobi operator are constant), thus both conditions are equivalent. 
Miguel Brozos-V\'azquez and Eugenio Merino
math.DG