The ultrafast modulation spectroscopy of a four-level system (UMSFS) has been studied in which the linewidths of the pump beams are either narrow or broad and it was found that for both cases a Doppler-free precision in the measurement of the energy-level difference can be achieved. In the condition that a beating occurs, the energy-level difference could exceed the laser linewidth, and the measurement accuracy could reach the same order of magnitude as the laser linewidth.