In the first three parts of this series a three-dimensional (3D) model was developed for transport and reaction of gaseous mixtures in a landfill. An optimization technique was also utilized in order to determine a landfill's spatial distributions of the permeability, porosity, the tortuosity factors, and the total gas generation potential, given a limited amount of experimental data. In the present paper we develop the model further by including the flow of both the leachate and the gases. A 3D dynamic model is developed that accounts for the generation of the four main gases of a landfill, along with the dissolved organic acids and the carbon in the presence of the leachate. The model is then utilized, through extensive numerical simulations, to study the effect of the various factors on the concentrations of the gases and the micro-organism. In particular, we demonstrate the strong effect of the heterogeneities of a landfill, represented by the spatial distribution of the local porosities, as well as an anisotropic distribution of the local permeabilities, on the behavior of a landfill, and in particular pressure buildup in it. (C) 2009 Elsevier Ltd. All rights reserved.