Аннотация:We discuss the amplitude of the B→l+l−l′ν′ decays and the differential decay rate d2Γ/dq2dq′2, q the momentum of the l+l− pair emitted from the electromagnetic vertex and q′ the momentum of the l′ν′ pair emitted from the weak vertex. For the relevant form factors, we construct dispersion representations in q2, which consistently take into account the Ward identity constraints at q2=0 and the contributions of light vector resonances. This allows a consistent description of the form factors in the range 0<q2≤1 GeV2 that saturates around 99% of the decay rate. The differential decay rate behaves at small q2 as dΓ(B→l+l−l′ν′)/dq2∝1/q2 in the limit ml′=0 but contains also more singular contribution of order ml′2/q4, which we take into account. For the case ml′≤ml, the latter may be neglected, and one obtains a mild logarithmic dependence of Γ(B→l+l−l′ν′) on ml. For the case ml≪ml′, however, the ml′2/q4 terms dominate the decay rate, leading to Γ(B→l+l−l′ν′)∼ml′2/ml2. We find the following features of the four-lepton B decays: i) The decay rates Γ(B→μ+μ−(μνμ,eνe)) are fully dominated by the region of light vector resonances q2≃Mρ2,Mω2. ii) The decay rate Γ(B→e+e−eνe) receives comparable contributions from the region near q2∼4me2 and from the resonance region. iii) One finds a strong enhancement of the decay rate Γ(B→e+e−μνμ)∼mμ2/me2, which is dominated by the region q2∼4me2 due to the terms O(mμ2/q4) in the differential distribution.