The second-order electron density fluctuation in homogeneous unmagnetized plasma is calculated by means of dynamical method. The third-order non-linear susceptibility resulting from interaction of three waves in plasma and the scattered power of K4=K1-K2+K3,ω4=ω1-ω2+ω3 is obtained. As compared with the normal non-coherent Thomson scattering, the scattered signal is very strong. So the coherent four-photon scattering can be taken as one of the important method in plasma diagnostics. It is showed that the result of reference 4 is a simplified form of the result obtained here.