  We report the observation of a dimer-dimer inelastic collision resonance for
ultracold Feshbach molecules made of bosonic sodium and rubidium atoms. This
resonance, which we attribute to the crossing of the dimer-dimer threshold with
a heteronuclear tetramer state, manifests itself as a pronounced inelastic loss
peak of dimers when the interspecies scattering length between the constituent
atoms is tuned. Near this resonance, a strong modification of the temperature
dependence of the dimer-dimer scattering is observed. Our result provides
insight into the heteronuclear four-body system consisting of heavy and light
bosons and offers the possibility of investigating ultracold molecules with
tunable interactions.
