We study the expected sensitivity at Belle and Belle II for four-body τ∓→X±l∓l∓ντ decays where l=e or μ and X=π, K, ρ and K∗ mesons. These decay processes violate the total lepton number (|ΔL|=2 ) and they can be induced by the exchange of Majorana neutrinos. In particular, we consider lifetimes in the accessible ranges of τN = 5, 100 ps and extract the limits on |VℓN|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(τ∓→X±l∓l∓ντ) ∼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(τ∓→X±l∓l∓ντ) ∼10−9−10−8. We use these sensitivities to set limits for the exclusion regions on the parameter space (mN,|VℓN|2) associated with the heavy neutrino; such that for a |VℓN|2∼O(10−5) at τN=100 ps, we find the bounds as 0.140