In this paper, an optimization method for arranging a four-coil WPT system is proposed to improve medium or long-distance power transfer efficiency. The system efficiency model with respect to the coil mutual inductance is established. Then the nonlinear constraints about power and distance are given, and the distance between adjacent coils is optimized through an optimization algorithm. Based on the optimization method, the system efficiency with identical coils and different coils is also compared. Finally, the simulation is carried out to show that the system efficiency can be improved from 56.42% under identical-coil condition to 67.23% under different-coil condition, where the distance between adjacent coils is optimized.