GaxIn1−xAsyP1−y epilayers closely lattice matched, Δa/a≲5×10−4, have been reproducibly grown over the whole range of composition (y=2.2x, 1>y>0) by chemical beam epitaxy. The relative sticking coefficient (or equivalently the incorporation efficiency into the solid) of arsenic to phosphorus, i.e., SAs/SP was between 1.5 and 3 depending on the material composition. Such values indicate a very efficient incorporation of phosphorous in this process. Very intense efficient luminescence peaks due to excitonic transitions with linewidths full width at half‐maximum as narrow as 3 meV were obtained. Such a linewidth corresponds closely to the intrinsic linewidth due to alloy broadening in GaInAsP alloys. Furthermore, the photoluminescence spectra revealed that the donor‐to‐acceptor pair recombination was nearly absent. Hall measurements on GaxIn1−xAsyP1−y epilayers lattice matched into InP at 300 and 77 K yielded electron mobility values that agreed closely with theoretical values calculated by using the one‐phonon model and the electronegativity difference as the alloy scattering potential for layers with doping levels between 1×1015 cm−3 and 1×1016 cm−3. The 77 K electron mobilities ranged from 2.2×104 to 6.7×104 cm2/V s depending on the quaternary composition.