A 4-channel wavelength division (de)multiplexer with low insertion loss and flat-top transmission spectra is presented based on the novel scandium (Sc)-doped aluminum nitride (Al1-xScxN) photonics platform for the first time, and an optimization method is proposed. The designed multiplexers are based on cascaded Mach-Zehnder interferometer (MZI) filters, using directional couplers with curved waveguides to reduce wavelength dependence. Particle swarm algorithm (PSO) is adopted to increase the efficiency of device optimization according to the characteristics of coarse wavelength division multiplexing (CWDM) applications. The transmission quality of four channels is improved by adjusting the structure of the device. For Sc doping concentrations of 0%, 9% and 23%, the designed multiplexers have 1-dB bandwidths as wide as about 15.6 nm and negligible insertion losses of less than 0.1 dB, showing "box-like" responses. The crosstalks of four channels are less than -30.6 dB.