We study the optical properties of a four-level Y system theoretically in $^{87}\rm{Rb}$ atoms using density matrix formalism. On account of wavelength mismatching effects in the system transparency window splits and enhanced absorption regions are observed on either side of the line centre. This enhanced absorption is associated with anomalous dispersion resulting in superluminal light propagation. We also explore the dressed state analysis in the system. The effect of mismatching factor on the group index and group velocity is discussed.
