  The Cabibbo-Kobayashi-Maskawa matrix element |V_ub| is not well determined
yet. It can be extracted from both inclusive or exclusive decays, like B --> pi
(rho) l nu-bar_l. However, the exclusive determination from B --> rho l
nu-bar_l, in particular, suffers from a large model dependence. In this paper,
we propose to extract |V_ub| from the four-body semileptonic decay B --> pi pi
l nu-bar_l, where the form factors for the pion-pion system are treated in
dispersion theory. This is a model-independent approach that takes into account
the pi-pi rescattering effects, as well as the effect of the rho meson. We
demonstrate that both finite-width effects of the rho meson as well as scalar
pi-pi contributions can be considered completely in this way.
