We present a complete investigation of crystalline, optical, and electrical properties of molecular‐beam‐epitaxial‐grown AlGaInAs lattice matched to InP covering the whole range of concentrations. Using the two indium cells method, we show very easy control of lattice matching of this quaternary system which can be written as (Al0.48In0.52As)z(Ga0.47In0.53As)1−z. X‐ray double diffraction profiles do not depend on the Al concentration and they show sharp diffraction linewidth. The PL full widths at half maximum are comparable to the narrowest reported. Transmission electron microscopy shows excellent crystallinity forzup to 0.60. Room‐temperature electron mobility higher than 4000 cm2/V s forzup to 0.40 is comparable to that of InP and is the best result reported up to now for these quaternary alloys. This system is thus quite suitable for microwave and optical devices applications.