Although phase-change materials (PCMs) have been studied for more than 50 years, temperature-dependent characterization of the phase transition dynamics remains challenging due to the lack of nanosecond-nanoscale thermometry. In this article, we utilize the four-terminal, indirectly heated phase-change switch (IPCS), which was originally designed for nonvolatile radio frequency (RF) applications, as an ultrafast electrothermal platform to study PCM. We propose a novel experimental setup that allows nanosecond probing of the transient resistance of the PCM, beyond the melting temperature (>1100 K), due to the built-in electrical isolation between the PCM path and the thermal actuation path of the IPCS. The embedded metallic heater can induce reversible phase transitions between the crystalline and amorphous phases of the PCM. Our platform enables simultaneous measurements of the dynamics of PCM resistance (as a probe for the phase of the material) and heater temperature, during the application of heating pulses. Furthermore, we map the surface temperature of the IPCS at steady state by scanning thermal microscopy (SThM) and show the effect of cooling by electrodes in devices with overlap between the heater and PCM contacts. Our method can be used to study chalcogenides and other amorphous semiconductors for reconfigurable electronics and neuromorphic hardware.