  Recent studies of novel four-body lepton number violating decays of $\tau$
leptons and neutral $B$ mesons are summarized and updated. These decays are
assumed to be enhanced by the exchange of resonant Majorana neutrinos. It is
shown that the $\tau^- \to \pi^+l^-l^-\nu_{\tau}$ decay channels, with $l=e$ or
$\mu$, can provide stronger constraints on the mixing vs. mass parameter space
of resonant Majorana neutrinos than analogous three-body decays of charged $B$
mesons.
