We report the results of high-sensitivity 4.8 GHz observations of four southern contact-binary stars (AE Phe, YY Eri, RW Dor, and V757 Cen) made with the Australia Telescope Compact Array. None of these stars were detected above a 4σ level of 0.4 mJy. A comparison of the radio emission properties of contact binaries (derived from this study and from the literature) to those of short-period RS CVn stars, suggests that a decrease in the dynamo efficiency of binary stars as they approach contact may be required to explain the lower intrinsic radio luminosity of contact binaries