We prove that proper biharmonic hypersurfaces with constant scalar curvature in Euclidean sphere S-5 must have constant mean curvature. Moreover, we also show that there exist no proper biharmonic hypersurfaces with constant scalar curvature in Euclidean space E-5 or hyperbolic space H-5, which give affirmative partial answers to Chen's conjecture and the Generalized Chen's conjecture.