Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni3IIILnIII} star‐type complexes Ln=Gd (1), Dy (2) to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute ∢NiOGd bond angles present in 1 instigate a significant interaction between the 4fxyz orbital of the GdIII ion and 3d${{_{x{^{2}}- y{^{2}}}$ orbital of the NiII ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next‐nearest‐neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI‐SO calculations performed on complex 2 suggest that the octahedral environment around the DyIII ion is neither strong enough to stabilize the mJ |±15/2〉 as the ground state nor able to achieve a large ground‐state–first‐excited‐state gap. The ground‐state Kramers doublet for the DyIII ion is found to be the mJ |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the JNiDy interaction as −1.45 cm−1. The strong Ni⋅⋅⋅Dy and next‐nearest‐neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero‐field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar Dy(AlMe4)3 highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established. Quantum tunneling: DFT and ab initio calculations suggest that both Ni⋅⋅⋅Dy and 1, 3 Ni⋅⋅⋅Ni (see figure) interactions help to quench the QTM behavior in {3d–4f} single‐molecule magnets.