InxAlyGa1-xN quaternary alloys with different In and Al compositions were grown by metalorganic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. It was observed that the dominant optical transition at low temperatures in InxAlyGa1-xN quaternary alloys was due to localized exciton recombination, while the localization effects in InxAlyGa1-xN quaternary alloys were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Our studies have revealed that InxAlyGa1-xN quaternary alloys with lattice matched with GaN epilayers (y≈4.8x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensity (or quantum efficiency) in InxAlyGa1-x-yN quaternary alloys than that of GaN. The quantum efficiency of InxAlyGa1-xN quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. These results strongly suggested that InxAlyGa1-x-yN quaternary alloys open an avenue for the fabrication of many optoelectronic devices such as high efficient light emitters and detectors, particularly in the ultraviolet region.