  We compute the connected four-point correlation function of the primordial
curvature perturbation generated during inflation with standard kinetic terms,
where the correlation is established via exchange of a graviton between two
pairs of scalar fluctuations. Any such correlation yields a contribution to the
scalar trispectrum of the order of the tensor to scalar ratio r. This
contribution is numerically one order of magnitude larger than the one
previously calculated on the basis of scalar perturbations interacting at a
point and satisfies a simple relation in the limit where the momentum of the
graviton which is exchanged becomes much smaller than the external momenta. We
conclude that the total non-linearity parameter generated by single-field
models of slow-roll inflation is at maximum tauNL ~ r.
