Quaternary InAsxPySb1-x-y alloys nearly lattice-matched to InAs substrates have been successfully grown by gas source molecular beam epitaxy (GSMBE) with the composition covering the immiscibility region. Through high resolution X-ray diffractometry, we observed the compositional inhomogeneity in these alloys. Enhancement in the As incorporation in the growth can not only narrow the inhomogeneous broadening but also improve the surface morphology. Carrier recombination in band-tail states caused by the compositional inhomogeneity is attributed to the low-temperature PL emission in these samples. The PL peak energy is thus lower than the predicted band-gap energy. The energy discrepancy can be as large as 0.26 eV, and decreases dramatically to 36 meV as the As mole fraction increases to 0.681. For the high As mole fraction sample, band-to-band recombination is observed as the temperature is higher than 100 K. (c) 2006 Elsevier B.V. All rights reserved.