The tetranuclear NiII2LnIII2 complexes, [{L'2{Ni(MeOH)(μ-OAc)}2(μ3-MeO)2Ln2}, LnIII = YIII (1), GdIII (2), TbIII (3), and DyIII (4)], were prepared using a Schiff base ligand, H3L [H3L = 3-{(2-hydroxy-3-methoxybenzylidene)amino}-2-(2-hydroxy-3-methoxyphenyl)-2,3-dihydroquinazolin-4(1H)-one, where {L'}3- is the deprotonated open structure of H3L]. X-ray crystallographic analysis of 1-4 revealed that all the complexes crystallized in the orthorhombic (Pbcn) space group, and possessed an isostructural tetranuclear butterfly or defect dicubane like core. Direct current magnetic susceptibility measurements performed on 2-4 revealed that all these complexes show an intramolecular ferromagnetic exchange coupling. Well resolved zero-field out-of-phase signals in ac magnetic susceptibility measurements were observed only in the case of 3 (Ueff = 13.4 K; τ0 = 4.1(7) × 10-7 s). This was attributed to the comparatively strong NiII-TbIII magnetic exchange coupling. DFT and ab initio calculations were carried out on 1-4 to ascertain the nature of the ferromagnetic NiII-LnIII (JNi-Ln) and LnIII-LnIII (JLn-Ln) interactions. Magnetic anisotropy and magnetic relaxation mechanisms were discussed in detail for 3 and 4. Theoretical studies provide a rationale for the slow relaxation of magnetization in 3.