In this paper we report the successful incorporation of sulfate into the triple layer (n=3) Ruddlesden Popper system Sr4Fe3O10-δ, with characterisation of these materials by X-ray diffraction, Mössbauer spectroscopy, and conductivity measurements. Significantly, the incorporation of SO42- into the middle perovskite layer leads to the stabilization of this Ruddlesden Popper phase, which cannot be prepared without sulfate doping, thus highlighting the potential of oxyanion doping as a strategy to synthesise new phases of interest for solid oxide fuel cells.