We present a characterization method for polarization converters which improves the accuracy of traditional characterization methods significantly. An experimental demonstration of the method is presented on the InP-membrane-on-silicon (IMOS) platform. The design and fabrication of the polarization converter is discussed, as well as the simulated polarization conversion efficiency. A device of only 4 microns is shown to achieve 97.5%±0.5% polarization conversion, corresponding to an extinction ratio of -16±0.9 dB. The traditional characterization method is compared to the new 4-port method, and the accuracy is improved from up to 20% to 0.5%.