The formation mechanism for the regular tetrahedral structure of Li₄ cluster is proposed. The curve of the total energy versus the separation R between the two nuclei has been calculated by using the method of Gou's modified arrangement channel quantum mechanics (MACQM). The result shows that the curve has a minimal energy of -29.8279 a.u. at R=14.50a₀. When R approaches infinity the total energy of four lithium atoms has the value of -29.7121 a.u. So the binding energy of Li₄ with respect to four lithium atoms is the difference of 0.1158 a.u. for the above two energy values. Therefore the binding energy per atom for Li₄ is 0.029 a.u., or 0.7878 eV, which is greater than the binding energy per atom of 0.453 eV for Li₂, the binding energy per atom of 0.494 eV for Li₃ and the binding energy per atom of 0.632 eV for Li₅ calculated previously by us. This means that the Li₄ cluster may be formed stably in a regular tetrahedral structure of side length R=14.50a₀ with a greater binding energy.