Following earlier three-dimensional (3D) calculations, we present results of four-dimensional (4D) calculations on dissociative and diffractive scattering of H-2 from Pt(111) by extending the 3D model with a second degree of freedom parallel to the surface. A 4D potential energy surface (PES) is constructed by interpolating four 2D PESs obtained from density-functional theory calculations using the generalized gradient approximation and a slab representation of the metal surface. The 4D calculations show that out-of-plane diffraction is much more efficient than in-plane diffraction, providing a partial explanation for the paradox that diffraction experiments measure little in-plane diffraction, whereas experiments on reaction suggest the surface to be corrugated. Calculations for off-normal incidence of v=0 H-2 show that, in agreement with experiment, initial parallel momentum inhibits dissociation at low normal translational energies, and enhances reaction for higher energies. Our 4D calculations also show that the reaction of initial v=1 H-2 is vibrationally enhanced with respect to v=0 H-2, as was found in the 3D model, even though H-2+Pt(111) is an early barrier system.