We report on the mid-infrared optical spectroscopy of quaternary InGaAsSb solid solution systems lattice-matched to InAs(0 0 1) substrate. Fourier-transformed photoreflectance has been used to study the pseudomorphic InGaAsSb epilayers grown by liquid phase epitaxy. The alloy composition dependence of the spin–orbit splitting energy was experimentally obtained at 77 K and found to be clearly nonlinear with the negative bowing parameter C(Δ0) = −0.35 eV. The points of energy resonance between the fundamental gap E0 and the spin–orbit splitting band Δ0 were discovered for two different InGaAsSb alloy compositions.