  We present the results of a theoretical study of a four-level atomic system
in vee + ladder configuration using a density matrix analysis. The absorption
and dispersion profiles are derived for a weak probe field and for varying
strengths of the two strong control fields. For specificity, we choose energy
levels of $^{87}$Rb, and present results for both stationary atoms and moving
atoms in room temperature vapor. An electromagnetically induced absorption
(EIA) peak with negative dispersion is observed at zero probe detuning when the
control fields have equal strengths, which switches to electromagnetically
induced transparency (EIT) with positive dispersion (due to splitting of the
EIA peak) when the control fields are unequal. There is significant linewidth
narrowing in thermal vapor.
