We have studied the indium and gallium incorporation during growth by chemical beam epitaxy of GaInAs, GaInP and GaInAsP alloys. TMIn, TEGa, cracked AsH3 and PH3 sources were used. The indium incorporation ratio remains nearly constant over the range of parameters investigated. whereas the gallium incorporation depends on the indium concentration, on the phosphorus concentration and on the growth temperature. The gallium incorporation ratio decreases linearly with indium content of the grown layer and the effect of the phosphorus and of the growth temperature during epitaxy of quaternary alloys can be described by a linear interpolation between the behavior observed on the ternary alloys.