The conditions of the maximum refractive index with vanishing absorption and the effect of the finite bandidths of the probe and driving field on the index of refraction and the gain are analyzed based on a Raman driven four level system. The calculated results show that the maximum index of refraction with vanishing absorption can be achieved by changing the detuning between the frewuency of the driving field and the resonant atomic transition or by changing the intensity of the driving field. The finite bandwidths of the driving and the probe field decrease the gain and change the value of the index of refraction, especially of the driving field.