We report on Raman scattering by longitudinal optical phonons in In 1-y-zAlyGazAs (1-y-z=0.53) lattice matched to InP. The quaternary alloys were grown on (001) InP by molecular beam epitaxy. The phonon spectra exhibit three-mode behavior. The frequencies of AlAs- and GaAs-like modes vary linearly with the concentration of Al (or Ga) while the position of the InAs-like phonon remains nearly constant. The data show no evidence of alloy clustering.