The use of methylene-bridged calix[4]arenes in 3d/4f chemistry produces a family of clusters of general formula [Mn(4)(III)Le(4)(III)(OH)(4)(C4)(4)(NO3)(2)(DMF)(6)(H2O)(6)](OH)(2) (where C4 = calix[4]arene; Ln = Gd (1), Tb (2), Dy (3)). The molecular structure describes a square of Ln(III) ions housed within a square of Mn-III ions. Magnetic studies reveal that 1 has a large number of molecular spin states that are populated even at the lowest investigated temperatures, while the ferromagnetic limit S = 22 is being approached only at the highest applied fields. This, combined with the high magnetic isotropy, makes the complex an excellent magnetic refrigerant for low-temperature applications. Replacement of the isotropic Gd-III ions with the anisotropic Tb-III and Dy-III ions "switches" the magnetic properties of the cluster so that 2 and 3 behave as low-temperature molecular magnets, displaying slow relaxation of the magnetization.