This paper reports a theoretical analysis of the electronic structure and magnetic properties of a tetranuclear CuII complex, [Cu4(HL)4], which has a 4+2 cubane-like structure (H3L=N,N′-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine)). These theoretical calculations indicate a quintet (S=2) ground state; the energy-level distribution of the magnetic states confirm Heisenberg behaviour and correspond to an S4 spin–spin interaction model. The dominant interaction is the ferromagnetic coupling between the pseudo-dimeric units (J1=22.2 cm−1), whilst a weak and ferromagnetic interaction is found within the pseudo-dimeric units (J2=1.4 cm−1). The amplitude and sign of these interactions are consistent with the structure and arrangement of the magnetic Cu 3d orbitals; they accurately simulate the thermal dependence of magnetic susceptibility, but do not agree with the reported J values (J1=38.4 cm−1, J2=−18.0 cm−1) that result from the experimental fitting. This result is not an isolated case; many other polynuclear systems, in particular 4+2 CuII cubanes, have been reported in which the fitted magnetic terms are not consistent with the geometrical features of the system. In this context, theoretical evaluation can be considered as a valuable tool in the interpretation of the macroscopic behaviour, thus providing clues for a rational and directed design of new materials with specific properties.