Properties of quaternary InAlGaAs alloys prepared by gas source MBE growth have been investigated with high resolution X-ray diffraction rocking curves, photoluminescence and Hall measurements. X-ray rocking curves show that all the samples are well matched to InP substrate according to calibration data. The photoluminescence and Hall measurement show that the PL intensity, electron concentration and mobility decrease distinctly as Al composition increases. The group III compositions are determined from both photoluminescence and x-ray diffraction measurements, and agreed well with the designed values. The relationship between the designed Al compositions and measured values provides us a practical way for the precise composition control.