The Raman spectra of quaternary InAsxSbyP1-x-y epitaxial layers nominally lattice-matched to (1 0 0) n-type InAs substrates are reported. The phonon peaks are identified for alloys covering a wide composition range extending into the miscibility gap. Three-mode behaviour was obtained across part of the composition range. InP-like longitudinal optical (LO) and transverse optical (TO) phonons were observed over the entire compositional range, but InAs-like LO phonons were only observed at high arsenic concentrations and no InSb-like TO phonons were observed. Disorder-related phonon peaks were obtained for alloy compositions within the miscibility gap.