The structure, stability, and bonding of the complexes formed by the interaction of Mg4 clusters and first row Lewis bases, namely, ammonia, water, and hydrogen fluoride, have been investigated through the use of high-level G4 single-reference and CASPT2 multireference formalisms. The adducts formed reflect the high electrophilicity of the Mg4 cluster through electron density holes in the neighborhood of each metallic center. After the adduct formation, the metallic bonding of the Mg4 moiety is not significantly altered so that the hydrogen shifts from the Lewis base toward the Mg atoms lead to new local minima with enhanced stability. For the particular case of ammonia and water, the global minima obtained when all the hydrogens of the Lewis base are shifted to the Mg4 moiety have in common a very stable scaffold with a N or an O center covalently tetracoordinated to the four Mg atoms, so the initial bonding arrangements of both reactants have completely disappeared. The reactivity features exhibited by these Mg4 clusters suggest that nanostructures of this metal might have an interesting catalytic behavior.