GaxIn1−xAsyP1−ylayers were grown on InP substrates by gas‐source molecular beam epitaxy in order to study the electrical and optical properties of the quaternaries. Hall mobilities and carrier concentrations of Si‐ and Be‐doped quaternary layers were compared in the alloys for different wavelengths. Smaller hole concentrations were observed for the alloy compositions close to InP than for those close to GaInAs grown with the same Be cell temperature. In addition, the highest achievable hole concentration was found to be a function of the alloy composition. Possible reasons for the observations are discussed. The influence of pregrowth treatments on the measured Hall mobilities and carrier concentrations due to the changes in the substrate‐epilayer interfacial conductivity was studied. Room‐temperature photoluminescence results are also presented.