  Neutrinoless hadron Lepton Number Violating (LNV) decays can be induced by
virtual Majorana neutrino, which in turn indubitably show the Majorana nature
of neutrinos. Many three-body LNV processes and Lepton Flavour Violating (LFV)
processes have been studied extensively in theory and by experiment. As a
supplement, we here study 75 four-body LNV (LFV) processes from heavy
pseudoscalar $B$ and $D$ decays. Most of these processes have not been studied
in theory and searched for in experiment, while they may have sizable decay
rates. Since the four-body decay modes have the same vertexes and mixing
parameters with three-body cases, so their branching fractions are comparable
with the corresponding three-body decays. We calculate their decay widths and
branching fractions with current bounds on heavy Majorana neutrino mixing
parameters, and estimate some channels' reconstruction events using the current
experimental data from Belle.
