  We study the expected sensitivity at Belle and Belle II for four-body
$\tau^\mp \to X^\pm l^\mp l^\mp \nu_{\tau}$ decays where $l=e$ or $\mu$ and
$X=\pi$, $K$, $\rho$ and $K^*$ mesons. These decay processes violate the total
lepton number ($|\Delta L|=2$ ) and they can be induced by the exchange of
Majorana neutrinos. In particular, we consider lifetimes in the accessible
ranges of $\tau_N$ = 5, 100 ps and extract the limits on $|V_{\ell N}|^2$
without any additional assumption on the relative size of the mixing matrix
elements. For an integrated luminosity collected of 1 ab$^{-1}$ at Belle, we
found significant sensitivity on branching fractions of the order BR($\tau^\mp
\to X^\pm l^\mp l^\mp \nu_{\tau}$) $\sim 10^{-8}$. For an integrated luminosity
expected of 50 ab$^{-1}$ and intermediate luminosity of 10 ab$^{-1}$ at the
Belle II, we found significant sensitivity on branching fractions of the order
BR($\tau^\mp \to X^\pm l^\mp l^\mp \nu_{\tau}$) $\sim 10^{-9}-10^{-8}$. We use
these sensitivities to set limits for the exclusion regions on the parameter
space $(m_N, |V_{\ell N}|^2)$ associated with the heavy neutrino; such that for
a $|V_{\ell N}|^2 \sim \mathcal{O}(10^{-5})$ at $\tau_N = 100$ ps, we find the
bounds as $0.140 < m_N < 1.776$ GeV for $\tau^- \to X^+ e^- e^- \nu_\tau$ and
$0.245 < m_N < 1.671 $ GeV for $\tau^- \to X^+ \mu^- \mu^- \nu_\tau$.
