A supramolecular strategy has been applied to construct two tetranuclear lanthanide complexes for investigating the magnetic properties of individual lanthanide ions. The Ln(III) complexes (Ln = Dy, Tb) display field-induced slow magnetization relaxation, typical of single-molecule magnet behavior. The four lanthanide ions in the molecules are well separated by distances of ca. 9 Å, and thus the slow magnetization relaxation should be assigned to single-ion magnet (SIM) behavior. Therefore, the present complexes are novel supramolecular aggregates of four isolated SIMs.