  We report the experimental realization of a single-species atomic four-wave
mixing process with BEC collisions for which the angular distribution of
scattered atom pairs is not isotropic, despite the collisions being in the
$s$-wave regime. Theoretical analysis indicates that this anomalous behavior
can be explained by the anisotropic nature of the gain in the medium. There are
two competing anisotropic processes: classical trajectory deflections due to
the mean-field potential, and Bose enhanced scattering which bears similarity
to super-radiance. We analyse the relative importance of these processes in the
dynamical buildup of the anisotropic density distribution of scattered atoms,
and compare to optically pumped super-radiance.
