The configurational-bias Monte Carlo (CBMC) technique in the grand-canonical (mu VT) ensemble has been used to calculate the pure and binary adsorption equilibrium isotherms of propane and propylene in zeolite 4A (zeolite Na-LTA) at 298.15, 373, 423, and 473 K in the 0-500 kPa pressure range. The influence of sodium cations on the adsorption properties of zeolite 4A was investigated using the united atom (UA) force field in order to establish effective Lennard-Jones (LJ) parameters for interactions between the cations and the sp(2)-hybridized bond of propylene as well as the other cation-carbon interactions present in this system. A cation-free zeolite Linde type A (LTA) framework and the inclusion of sodium cations into the zeolite framework have been studied in this work. The quality of the parameters describing the carbon-sodium interactions was investigated by comparing with published experimental results for the cation-free LTA and by using other published LJ interaction parameters. Surprisingly, it was found that the sodium force field developed by Calero et al. [J. Am. Chem. Soc. 2004, 126, 11377] which is suitable for studying adsorption in other Na-contained zeolite frameworks, namely, FAU and MFI, does not work well for the Na-LTA system. It turns out that this is due to the interaction with sodium, as the simulations without cations agree very well with adsorption experiments on cation-free LTA. Our force field for zeolite 4A is in good agreement with recent experiments which show that the adsorbed amount of propane is much higher than reported previously. Good agreement of simulations with experiments was also obtained for adsorption of propane/propylene mixtures in zeolite 4A.