The population transfer of a four level system coupled with three lasers is investigated in terms fo dressed atom model, and two dressed states are found to be similar to the so called trapped state in three level system. The population trapping in the four level system is analyzed and an optical pumping method with high efficiency is proposed for pouplation transfer from both ground state and metastable state. The superiority of the method is demonstrated numerically when it is used in U isotopes separation.