We present a 1x4 reconfigurable demultiplexing filter based on cascaded thermally tunable silicon racetrack resonators with ultralow tuning powers. The use of free-standing silicon resonators with undercut structures significantly reduces the tuning power, with a figure of ~2.9 mW per free spectral range. Even with the presence of thermal crosstalk between two adjacent resonators, we demonstrate multiplexing functionality for channel spacings of 200 GHz, 100 GHz, and 50 GHz, with channel wavelengths aligned to International Telecommunication Union (ITU) grid specifications. Crosstalk values for 200 GHz and 50 GHz channel spacings are less than -20 dB and -11.5 dB, respectively. The total power to achieve this performance is in the range of 1.84 mW to 2.4 mW. Such low-power, compact, and reconfigurable filters are particularly useful in chip-scale optical interconnects.