A novel Si four-wavelength multiplexer (MUX) for 100/400GbE composed of (a)symmetric directional couplers and a rib-waveguide TE-TM mode converter is proposed and experimentally demonstrated. Two-lane signals are multiplexed as TE modes via symmetric directional coupler (DC) and other two-lane signals are multiplexed as TE modes via asymmetric directional couplers (ADCs). Finally, TE modes are converted to TM modes and the device acts as a 4-lane MUX. The proposed device is fabrication-tolerant due to the removal of the 1600-GHz filter used in conventional two-stage Mach-Zehnder MUXs, and the insertion loss is smaller than those of previously reported Si-based MUXs.