The synthesis, structure and magnetism of tetranuclear complexes, [Ln4(LH2)2(η1-Piv)2(η2-Piv)2(μ3-O2)2(H2O)2]·2MeOH (1, Ln = GdIII; 2,Ln = TbIII; 3, Ln = DyIII; and 4, Ln = ErIII) (LH4 = 6-((bis(2-hydroxyethyl)amino)-N′-(2-hydroxybenzylidene)picolinohydrazide) (piv = pivalate ion) are reported. Interestingly, the formation of these tetranuclear assemblies is assisted by two side-on coordinating peroxide ligands which bind in a μ3-η2:η2:η2 fashion. DC magnetic behaviour revealed the existence of a dominating but weak intramolecular antiferromagnetic interaction in the cases of 1, 2, and 4, while a small structural change in complex 3 turns this into a ferromagnetic interaction. We have investigated the magnetothermal behaviour of 1 which shows a magnetocaloric effect with the value of maximum entropy change of −ΔSm = 33.60 J K−1 kg−1 at 4 K (ΔH = 13–0 T). This is close to the calculated value of 36.45 J K−1 kg−1. The ac magnetic susceptibility study confirms zero-field out-of-phase magnetic susceptibility signals only for 3 which get well resolved on the application of a 1 kOe dc magnetic field. The energy barrier for the Orbach process of spin reversal was found to be 23 K (τ0 = 9 × 10−7 s). We also report detailed theoretical studies to rationalize and understand the observed magnetic behaviour.