Multifrequency timing measurements of four pulsars were used to search for propagation delays predicted by the radius-to-frequency mapping (RFM) model of pulsar radio emission. The RFM model specifies that emissions at different frequencies are produced at different altitudes in the pulsar magnetosphere. An analysis including aberration, retardation, and magnetic sweepback effects showed that the 47 and 4800 MHz emission zones were separated by no more than 200 km