We present the results of our all-electron density-functional calculations on the magnetic anisotropy of the [Fe-4(sap)(4)(MeOH)(4)] and [Fe-4(sae)(4)(MeOH)(4)] polynuclear complexes. Our calculations, which predict that only the second complex is a single-molecule magnet (with a magnetic anisotropy energy barrier of 5.6 K), are in qualitative agreement with the experimental data. The analysis of the projected anisotropies of each Fe-II ion, together with a study of the variation of the D value as a function of several geometrical parameters, allows us to qualitatively understand the different magnetic behaviors of both complexes. In addition to this, we also present a simple rule based on the analysis of the molecular orbitals of the system that allows us to predict how to enhance (by a factor of 6, approximately) the magnetic anisotropy barrier of these systems. Specifically, we will show that, for high-spin Fe-II ions, the local easy axis of magnetization is perpendicular to the plane defined by the Fe-II-d orbital which is doubly occupied. If similar rules were found for other metal ions, rational synthetic strategies to control magnetic anisotropy could be established.