An ultra-compact multimode-interference (MMI)-based 1 × 4 nano-photonic switch is demonstrated by employing silicon thermo-optical effect on SOI platform. The device performance is systematically characterized by comprehensively investigating the constituent building blocks, including 1 × 4 power splitter, 4 × 4 MMI coupler and groove-isolated thermo-optical heaters. An instructive model is established to statistically estimate the required power consumption and investigate the influence of the power imbalance of the 4 × 4 MMI coupler on the switching performance. At the designed wavelength of 1550 nm, the average insertion loss of different switching states is 1.7 dB, and the transmission imbalance is 1.05 dB. The worst extinction ratio and crosstalk of all the output ports reach 11.48 dB and -11.38 dB, respectively.