Using X-ray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC), we have investigated the reaction between MgB4 and Mg in its solid, liquid, and gas physical states. The XRD results indicate that the reaction of MgB4+Mg=2MgB(2) always occurs independently of the different states of Mg. DSC measurements show that the solid-solid reaction MgB4(s)+Mg(s)=2MgB(2)(s), the Mg melting Mg(s)-> Mg(l), and the Mg volatilization occur in turn with increasing temperature for mixed (MgB4+Mg) powder with a nominal stoichiometry of MgB2. SEM observations indicate that the resulting synthesized MgB2 samples, obtained from the MgB4+Mg mixtures, have a denser microstructure than those obtained directly from Mg+B mixtures. On the basis of XRD, DSC, and SEM results, a kinetic model for the reaction between MgB4 and Mg is proposed.