We present a theoretical study on the nonlinear behaviors of the electromagnetically induced transparency
resonance subject to two microwave driving fields in a four-level atom system. The probe absorption spectrum is
obtained by solving numerically the relevant equations of density matrix. It is shown that there are two pairs of the EIT
windows in the probe absorption spectrum. The two pairs of EIT windows have symmetry with respect to the resonance
frequency of the probe field, and the separation is equal to the Rabi frequency of the resonant microwave driving field.
But in each pair, the splitting of two EIT windows is dominated to the strength of detuning microwave driving field.