The mixed-valent tetramanganese MnIII3MnIV (Mn4) cubane clusters have been at the forefront of molecular magnetism and biomimetic catalysis research for decades. Incorporating robust polyoxometalates to Mn4 cubanes significantly improves their stability and aqueous solubility, while providing a great platform for studying their deposition onto selected surfaces during device fabrication. In this work, we discovered that the terminal carboxylate ligands in these polyoxometalate-based [MnIII3MnIVO4] magnetic clusters can be partially or completely replaced by inorganic phosphate/polyphosphate groups. This replacement leads to oligomeric aggregates of the Mn4 clusters. The magnetic data of the monomeric and oligomeric Mn4 clusters suggested that the introduction of inorganic phosphate bridges may not alter the S = 9/2 ground state of individual Mn4 clusters, although different magnetic behaviors, especially at low temperatures, were observed primarily because of intercluster interactions.