In this paper we study the quantum interference effects of a quasi-Λ-type four-level atom system interacting with two light fields. When the probe field is weak, our calculations show that along with the change of Rabi frequencies of the driving field, the system appears to be hole burning in the dispersion curve,and two big electromagnetic induced transparency windows with approximately the same width are demonstrated. In addition, the evolution of dispersion with the Rabi phase of probe field shows optical bistability.