Four tetra-nuclear lanthanide complexes: {[Ln₄(L)₆(tmhd)₄(μ₃-OH)₂]·mH₂O} (Ln = Gd (1, m = 0), Tb (2, m = 3), Dy (3, m = 0), Ho (4, m = 0); L = 5-((pyridin-4-ylmethylene)amino)quinolin-8-ol, tmhd = 2,2,6,6-tetramethylheptane-3,5-dione) were fabricated and structurally characterized. Compounds 1–4 are isostructural and belong to the monoclinic system with space group P2₁/n. The cores of the complexes contain a tetranuclear arrangement of Ln^III ions, which is bridged by the two pyramidal μ₃-OH⁻ ions, and the four Ln^III ions are precisely coplanar. The magnetic study reveals that 1 exhibits cryogenic magnetic refrigeration property (−ΔS_m = 20.85 J K⁻¹ kg⁻¹), whereas compound 3 exhibits slow relaxation of the magnetization.