The group III (B, Al, Ga, In) nitride quaternary alloy systems are potentially useful for ultraviolet, blue, and green light-emitting devices, or high-temperature, high-power, and high-frequency electronic devices. There have been significant challenges to the epitaxial growth of these alloys and we have investigated the unstable mixing region in the phase field. The existence of an unstable mixing region is predicted based on a strictly regular solution model. The interaction parameter used in our model is analytically obtained by the valence force field model modified for wurtzite structures. From our calculations, among the group III nitride quaternary alloy systems, we find that InGaAlN system has the narrowest unstable mixing region, and that the BInAlN system has the widest unstable mixing region. The calculated interaction parameters which are the important to predict the unstable mixing region agree well with the best-fit line of experimental results for various alloy systems.