Abstract We use the Generalized Quasi‐Chemical Approach (GQCA) combined with ab initio ultrasoft pseudopotential calculations within density functional theory in order to obtain the structural and electronic properties of Al x Ga y In 1– x – y X (X = As, P or N) quaternary alloys in the zincblende structure. Results for the bond lengths show that their variations with composition are approximately linear and that they do not deviate much from the values of the corresponding binary compounds. For the variation of the band gaps, we obtain a bowing parameter b = 0.26 eV for the (Ga 0.47 In 0.53 As) z (Al 0.48 In 0.52 As) 1– z quaternary alloy lattice matched to InP, in very good agreement with experimental data. In the case of AlGaInN, a bowing parameter of 0.22 eV is obtained for zincblende AlGaInN lattice matched to GaN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)