  We show that the dispersion properties imposed by an external periodic
potential ensure both energy and quasi-momentum conservation such that
correlated pairs of atoms can be generated by four wave mixing from a
Bose-Einstein condensate moving in an optical lattice potential. In our
numerical solution of the Gross-Pitaevskii equation, a condensate with initial
quasi-momentum k_0 is transferred almost completely (>95%) into a pair of
correlated atomic components with quasi-momenta k_1 and k_2, if the system is
seeded with a smaller number of atoms with the appropriate quasi-momentum k_1.
