  We study the propagation of a quantum probe light in an ensemble of
"3+1"-level atoms when the atoms are coupled to two other classical control
fields. First we calculate the dispersion properties, such as susceptibility
and group velocity, of the probe light within such an atomic medium under the
case of three-photon resonance via the dynamical algebra method of collective
atomic excitations. Then we calculate the dispersion of the probe light not
only under the case that two classical control fields have the same detunings
to the relative atomic transitions but also under the case that they have the
different detunings. Our results show in both cases the phenomenon of
electromagnetically induced transparency can accur. Especially use the second
case, we can find two transparency windows for the probe light.
