Absorption, dispersion, group index, and delay/advance time versus Δp/Γ such that γ41 = 1Γ, γ21,32,43 = 0.02Γ, ω0 = 103Γ, Ω1 = Ω3 = 1Γ, Ω2 = 2Γ, Δ1 = Δ3 = 0Γ, η1 = η3 = 1, VD = 10Γ [dotted green], 15Γ [dashed blue], 20Γ [solid red].