The hot deformation behavior of as-cast Mg-4Sn-2Ca (TX42) alloy has been studied using compression tests in the temperature range of 300 C to 500 C, and strain rate range of 0.0003 s^-1 to 10 s^-1. Based on the flow stress data, a processing map has been developed, which exhibited two domains of dynamic recrystallization in the temperature and strain rate ranges: (I) 300 C to 380 C and 0.0003 s^-1 to 0.001 s^-1, and (II) 400 C to 500 C and 0.004 s^-1 to 6 s^-1. While hot working may be conducted in either of these domains, the resulting grain sizes are finer in the first domain than in the second. The apparent activation energy values estimated by kinetic analysis of the temperature and strain rate dependence of flow stress in the domains 1 and 2 are 182 kJ/mol and 179 kJ/mol, respectively. Both the values are much higher than that for self-diffusion in pure magnesium, indicating that the thermally stable CaMgSn particles in the matrix cause significant back stress during the hot deformation of this alloy. The alloy exhibits a regime of flow instability at lower temperatures and higher strain rates, which manifested as flow localization.