A wideband terahertz fourth-harmonic mixer with low local oscillator (LO) power requirement based on anti-parallel planar Schottky diodes is proposed. In order to realize best performances of the mixer, the relations between the parasitic elements of Schottky diodes and the mixer’s performances are analyzed. The main parameters of Schottky diodes are optimized to reduce the optimum LO power of the fourth-harmonic mixer. Measured results show that the conversion loss of the proposed mixer based on specialized diodes is 14.2 ~ 20 dB within a wide band from 340 to 490 GHz with optimal LO power of 7 mW, while the noise temperature is 4020~ 17100 K in this frequency range.