  We present a theoretical analysis of the spatial shape of two symmetric
signals of degenerate four-wave mixing induced by Gaussian beams in a thin
sample of two-level atoms. Our calculations take into account the full spatial
and spectral dependencies of the relevant nonlinear susceptibilities that
govern the two processes. This reveals two interesting effects. The first one
is that the total power of incident beams affects the transverse profile of the
four-wave mixing signals at the medium exit and their free propagation. The
second one is the influence of the spectral characteristics of the medium on
the longitudinal profile of both generated signals upon free propagation. We
argue that the first effect can be seen as the saturation of the medium in
regions of higher intensity, while the second can be understood as the result
of a nonlinear contribution to the refractive index inside the atomic medium.
These effects can be symmetric between the two signals, with asymmetries
induced by different detunings from resonance of the incident fields.
