The low-temperature topotactic reduction of Sr3Fe2-xCoxO5Cl2 oxychloride phases with LiH allows the preparation of phases of composition Sr3Fe2-xCoxO4Cl2 (0 <= x <= 1). The reduced phases adopt body-centered tetragonal structures which are isostructural with Sr3Fe2O4Cl2 and contain square-planar (Fe/Co)O-4 centers connected into apex-linked sheets, analogous to the CuO2 sheets present in superconducting cuprate phases. As the cobalt concentration in Sr3Fe2-xCoxO4Cl2 is increased the antiferromagnetic order of the Sr3Fe2O4Cl2 host phase is suppressed, ultimately leading to spin-glass behavior, at low temperature, in Sr3Fe2-xCoxO4Cl2 phases with x >= 0.8. The limited influence of cobalt substitution on the reactions which form the Sr3Fe2-xCoxO4Cl2 phases is discussed and contrasted to that of the related SrFeO3-delta-SrFeO2 system.