The effect of an intense pump field and the chemical concentration of the solution on the forward four-wave mixing signal of molecular systems with an inhomogeneous distribution of resonance frequencies, has been studied. A perturbative treatment to all orders in the pump and to first order in the probe and signal beams, including the propagation of the fields as the variable to be studied, was used. The present results are compared to local regimes of propagation and to the case of a homogeneously distributed spectral line. The behaviour of the non-linear signal, characterized by tridimensional intensity surfaces, is explained in terms of saturation and/or absorption processes taking place under the present study conditions.