Recent experimental literature reports the solid state electrolyte properties of Li4SnS4 and Li4SnSe4, identifying interesting questions regarding their structural details and motivating our first principles simulations. Together with Li4GeS4, these materials are all characterized by the orthorhombic space group Pnma and are found to be isostructural. They have a ground state crystal structure (denoted Li4SnS40) having interstitial sites in void channels along the c-axis. They also have a meta-stable structure (denoted Li4SnS*(4)) which is formed bymoving one fourth of the Li ions from their central sites to the interstitial positions, resulting in a 0.5 a contraction of the a lattice parameter. Relative to their ground states, the meta-stable structures are found to have energies 0.25 eV, 0.02 eV, and 0.07 eV for Li4GeS*(4), Li4SnS*(4), and Li4SnSe*(4), respectively. Consistent with these simulation results, the ground state forms for Li4GeS40, Li4SnS40 and Li4SnSe40 and the meta-stable form for Li4SnS*(4) have been reported in the experimental literature. In addition, simulations of Li ion migration in these materials are also investigated. (C) The Author(s) 2017. Published by ECS. All rights reserved.