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We discuss the amplitude of the $B\to l^+l^-l'\nu'$ decays and thedifferential decay rate $d^2\Gamma/dq^2dq'^2$, $q$ the momentum of the $l^+l^-$pair emitted from the electromagnetic vertex and $q'$ the momentum of the$l'\nu'$ pair emitted from the weak vertex. For the relevant form factors, weconstruct dispersion representations in $q^2$ which consistently take intoaccount the Ward identity constraints at $q^2=0$ and the contributions of lightvector resonances. This allows a reliable description of the form factors inthe range $0 < q^2 \le 1$ GeV$^2$ that saturates around 99% of the decay rate.The differential decay rate behaves at small $q^2$ as $d\Gamma(B\to l^+l^-l'\nu')/dq^2\propto 1/q^2$ in the limit $m'_l=0$, but contains also moresinglular contribution of order ${m'^2_l}/q^4$, which we take into account. Forthe case $m_l'\le m_l$, the latter may be neglected and one obtains a mildlogarithmic dependence of $\Gamma(B\to l^{+}l^{-}l'\nu')$ on $m_l$. For thecase $m_l\ll m'_l$, however, the ${m'^2_l}/q^4$ terms dominate the decay rateleading to $\Gamma(B\to l^{+}l^{-}l'\nu')\sim m'^2_l/m^2_l$. We find thefollowing features of the four-lepton $B$-decays: (i) The decay rates$\Gamma\left(B\to \mu^+\mu^-(\mu \nu_\mu,e\nu_e)\right)$ are fully dominated bythe region of light vector resonances $q^2\simeq M_\rho^2, M_\omega^2$; (ii)The decay rate $\Gamma(B\to e^{+}e^{-}e \nu_e)$ receives comparablecontributions from the region near $q^2\sim 4m_e^2$ and from the resonanceregion; (iii) One finds a strong enhancement of the decay rate $\Gamma(B\toe^+e^- \mu \nu_\mu)\sim m_\mu^2/m_e^2$ which is dominated by the region$q^2\sim 4m_e^2$ due to the terms $O(m_\mu^2/q^4)$ in the differentialdistribution.

Theoretical analysis of the leptonic decays $B\to \ell \ell \ell'\bar\nu_{\ell'}$