We report on Raman scattering by longitudinal optical phonons in In1-x-yGaxAlyAs quaternary alloys lattice-matched to InP(with x + y congruent to 0.47). The quaternary alloy samples were grown as epilayers on InP(001) substrates by molecular beam epitaxy. The Raman phonon spectra show a three-mode behaviour involving the InAs-, GaAs- and AlAs-like longitudinal optic phonon modes. The frequencies of GaAs- and AlAs-like modes vary linearly with the concentration of the Ga (or Al) while the position of the InAs-like phonon remains nearly constant. We show that the ratio of intensities of the modes is proportional to the corresponding ratio of their compositions. Disorder activated modes in the acoustic and optic regions due to the disorder in atomic arrangements of group III elements have also been observed. The mode frequencies have been modeled using the extended form of the Random Cell lso-Displacement, model (RCI), which provides a good description of the experimental results.