The long wavelength optical phonons in Ga x In 1- x As y Sb 1- y MBE epitaxial layers were investigated by Raman scattering spectroscopies and FIR reflection spectra. The four mode behavior and modes below 180cm -1 related with the interaction between the third nearest atoms in Ga x In 1- x As y Sb 1- y quaternary mixed crystals were observed from the Raman data, and the related phenomenon possibly arising from the multi phonon absorption process was observed from the FIR and Raman data.