  Four-wave mixing and walk-off between two optical beams are investigated for
fo cusing Kerr media. It is shown that four-wave mixing reinforces the
self-focusi ng of mutually-trapped waves by lowering their power threshold for
collapse, on ly when their phase mismatch is small. On the contrary, walk-off
inhibits the c ollapse by detrapping the beams, whose partial centroids
experience nonlinear o scillations.
