We have studied theoretically a beat effect in a four-level system due to fifth-order optical polarization, and considered the cases that the pump beams have either narrowband or broadband linewidth. We have found that the accuracy for the energy-level splitting meaurement is determined by the homogeneous linewidths of the optical transitions. That is to say, this technique can achieve Doppler-free precision in the measurement of the eaergy level splitting between two excited states which are dipolar forbidden from the ground state.