The mixed-valent tetramanganese (Mn3MnIV)-Mn-III (Mn-4) cubane clusters have been at the forefront of molecular magnetism and biomimetic catalysis research for decades. Incorporating robust polyoxometalates to Mn-4 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 [(Mn3MnO4)-Mn-III-O-IV] magnetic clusters can be partially or completely replaced by inorganic phosphate/polyphosphate groups. This replacement leads to oligomeric aggregates of the Mn-4 dusters. The magnetic data of the monomeric and oligomeric Mn-4 clusters suggested that the introduction of inorganic phosphate bridges may not alter the S = 9/2 ground state of individual Mn-4 clusters, although different magnetic behaviors, especially at low temperatures, were observed primarily because of intercluster interactions.