Using the Optical Stochastic Bloch equations (OSBE), and considering a two-level molecular system interacting with a thermal reservoir (modeled as an Ornstein-Uhlenbeck process) in four-wave mixing spectroscopy, we derive analytical expressions for the nonlinear optical properties (absorptive and dispersive processes). Here, we have widened the previous formalism [(J.L. Paz et al., Phys. Lett. A 226 (1997) 365)] to consider the solvent effects and high-pump-power in the four-wave mixing (FWM) technique. The saturation effects introduced by including high-pump-power are explicitly considered. These results are very useful when laser-beams of moderate or high-intensity are employed for the determination of relaxation mechanisms in molecular systems interacting explicitly with their environment.