Hi Andy, My 3 ferrets, Zen, Lillie, and Chibi comfortably reside in a FN model 42. I don't suggest housing anymore ferrets than that, in this model of the FN. I've purchased the add-on level making it the FN model 143, but haven't assembled it yet. I'm sure that the FN Model 143 can easily house 4 ferrets, but no more. HTH Randie [Posted in FML 6362]