The controlled manipulation of electronic and magnetic states in single-molecule magnets (SMMs) is crucial for their implementation in molecular electronics, spintronics, and quantum computation. In typical SMMs, key properties like magnetic anisotropy and slow magnetic relaxation are imposed by complex ligand shells, whose bulky and three-dimensional structures hamper efficient manipulation of the molecular magnetism by chemical methods. This work demonstrates highly selective redox doping of an Fe₄ nanomagnet on a Pb(111) surface using lithium atoms. Scanning tunneling microscopy, x-ray absorption spectroscopy, and ab initio calculations reveal the cooperative incorporation of three Li atoms per Fe4 molecule, resulting in a selective, threefold reduction of its iron-based magnetic core. The doping modifies the intramolecular exchange interaction, turning from antiferromagnetic to ferromagnetic, and changes the molecular magnetic anisotropy from easy-axis to easy-plane. This study demonstrates successful chemical redox doping of individual polynuclear molecular magnets, exploits a rare showcase of cooperative binding, and highlights a route for tuning magnetic properties of complex SMMs.