The phase diagram of the first layer of 4He adsorbed on a single graphene sheet has been calculated by a series of diffusion Monte Carlo calculations including corrugation effects. Since the number of C-He interactions is smaller than in graphite, the binding energy of 4He atoms to graphene is reduced approximately 13.4 K per helium atom. Our results indicate that the phase diagram is qualitatively similar to that of helium on top of graphite. A two-dimensional liquid film on graphene is predicted to be metastable with respect to the commensurate solid but the difference in energy between both phases is very small, opening the possibility of such a liquid film to be experimentally observed.