  We compute the fourth virial coefficient of a binary nonadditive hard-sphere
mixture over a wide range of deviations from diameter additivity and size
ratios. Hinging on this knowledge, we build up a $y$ expansion [B. Barboy and
W. N. Gelbart, J. Chem. Phys. {\bf 71}, 3053 (1979)] in order to trace the
fluid-fluid coexistence lines which we then compare with the available
Gibbs-ensemble Monte Carlo data and with the estimates obtained through two
refined integral-equation theories of the fluid state. We find that in a regime
of moderately negative nonadditivity and largely asymmetric diameters, relevant
to the modelling of sterically and electrostatically stabilized colloidal
mixtures, the fluid-fluid critical point is unstable with respect to
crystallization.
