Accurate GIA models are required for correcting measurements of mass change in Antarctica and for improving our knowledge of the sub-surface, especially in areas of large current ice loss such as the Amundsen Sea Embayment. There, seismic and gravity data suggests lateral differences in viscosity. Furthermore, mantle flow laws allow for time-varying viscosity. In this study we investigate whether spatial and temporal variations in viscosity (4D viscosity) have significant effects on the measured uplift in the region. We use a finite element model with composite rheology consisting of diffusion on and dislocation creep, forced by an ice deglaciation model starting in 1900. We use its uplift predictions as synthetic observations to test the performance of 1D model inversion in the presence of viscosity variations. Introducing time-varying viscosity results in lower viscosity beneath the load and a more localized uplift pattern. We demonstrate that the background stress from earlier ice load changes, can increase and decrease the influence of stress-induced viscosity changes. For the ASE, fitting 1D models to 3D model uplift results in a best fitting model with viscosity that is equal to the average of a large contributing area, while for 4D the local viscosity is more crucial. 1D models are statistically indistinguishable from 3D/4D models with current GPS stations. However, 3D and 4D models should be taken into account when accurate uplift and gravity rate patterns are needed for correcting satellite measurements or predicting relaxation times, as uplift can differ up to 45\% compared to 1D models.