In this theoretical investigation of the miscibility in quaternary alloys, we stay in the regular solution model and determine the interaction parameters from the enthalpy of mixing calculated using a modified Stillinger–Weber (SW) potential for III‐nitride ternary alloys. From our calculation, the values of the interaction parameters are 6.605, 10.523, and 0.677 kcal mol−1, respectively, for InGaN, InAlN, and AlGaN at x = 0.5 (where x is the In and Al fraction in InGaN, InAlN, and AlGaN, respectively). These values are in good agreement with those predicted by the improved delta lattice parameter (DLP) model for nitride ternary alloys. To determine the unstable regions characterized by the spinodal decomposition of the quaternary alloy InxAlyGa1−x−yN, we consider the composition ranges of 0.06 ≤ x ≤ 1 and 0 ≤ y ≤ 0.94. Our calculations show that the phase separation temperature increases with the increase of indium concentration. For In concentration lower than 9.4%, the quaternary alloy is miscible independent of the Al and Ga compositions in the usual growth temperature range for metalorganic vapor phase epitaxy.