Using a fused phthalocyaninato ligand to control the spatial arrangement of Tb-III moieties in Tb-III single-molecule magnets (SMMs), we could control the dipole-dipole interactions in the molecules and prepared the first tetranuclear Tb-III SMM complex. [Tb(obPc)(2)]Tb(Fused-Pc)Tb[Tb(obPc)(2)] (abbreviated as [Tb-4]; obPc = 2,3,9,10,16,17,23,24-octabutoxyphthalocyaninato, Fused-Pc = bis(7(2),8(2),12(2),13(2),17(2),18(2)-hexabutoxytribenzo[g,l,q]-5,10,15,20-tetraazaporphirino} [b,e]benzenato). In direct-current magnetic susceptibility measurements, ferromagnetic interactions among the four Tb3+ ions were observed. In [Tb-4], there are two kinds of magnetic dipole-dipole interactions. One is strong interactions in the triple-decker moieties, which dominate the magnetic relaxations, and the other is the weak one through the fused phthalocyaninato (Pc) ligand linking the two triple-decker complexes. In other words, [Tb-4] can be described as a weakly ferromagnetically coupled dimer of triple-decker Tb-2(obPc)(3) complexes with strong dipole dipole interactions in the triple-decker moieties and weak ones through the fused phthalocyaninato ligand linking the two triple-decker complexes. For [Tb-4], dual magnetic relaxation processes were observed similar to other dinuclear (TbPc)-Pc-III complexes. The relaxation processes are due to the anisotropic centers. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole-dipole (f-f) interactions between the Tb3+ ions in the systems. Through a better understanding of the magnetic dipole-dipole interactions obtained in these studies, we have developed a new strategy for preparing Tb-III SMMs. Our work shows that the SMM properties can be fine-tuned by introducing weak intermolecular magnetic interactions in a controlled SMM spatial arrangement.