Dielectric measurements have been performed on a series of samples consisting of poly-(acrylonitrile) (PAN) mixed with the plasticizers ethylene carbonate (EC) and propylene carbonate (PC) in various proportions. The spectra obtained show characteristic features such as a linear ln() variation, a horizontal inflection point and double minima, which are predicted within the A₄-scenario of the mode-coupling theory. We find very good agreement with the theoretical results in the relevant frequency range. The experimental spectra are found to satisfy a scaling behaviour, and the resulting master curves are in good agreement with the theoretical predictions.