The effects of phase fluctuation on absorption,dispersion and population difference in an open four-Level atomic system have been analyzed by using the numerical simulation from the steady analytical solution. It is shown that variation of the linewidth cannot change the property of the inversionless lasing of the system;increasing R_L reduces the magnitude of the gain and requires a larger Ω value for the onset of the gain;when the exit rate is small,effects of the finite linewidth R_L,on the gain,dispersion and population difference is very weak;the system can still get a high refractive index without absorption when the linewidth does not equal to zero.