A series of hydrogenous and deuterated four-arm polystyrene stars with low polydispersity have been synthesized by anionic polymerization. Self-diffusion data have been obtained using helium-3 nuclear reaction analysis, and rheology has been employed to obtain the zero shear viscosity. Both diffusion and viscosity data have been compared to results predicted by recent theories in order to test the alternative models of diffusion cases, i.e., arm retraction followed by the core hopping either a diluted or an undiluted tube diameter. The exponential dependence on molecular weight can be conveniently removed by taking the product of the self-diffusion and the viscosity, making it possible to determine which model is more appropriate. Our data show good agreement with the undiluted tube model over a range 3.5 > N/N-e < 9. At low molecular weight the data show the model failing at N/N-e < 3, in agreement with theoretical predictions.