We proposed a novel scheme for the fourth order harmonic generation with two Mach-Zehnder (MZ) optical modulators. High extinction ratio intensity modulation technique with active trimmers is used for high spurious suppression ratio. Each modulator has a pair of trimmers in the arms of the MZ structure. The trimmers can compensate amplitude imbalance in the MZ structure, and suppress residual carrier in double sideband suppressed carrier (DSB-SC) modulation. A photonic local signal of 42GHz was generated from a 10.5GHz modulating signal, where spurious component intensity was 41.8dBc without using any optical or electric filters for spurious suppression. The upper limit of modulating frequency in the state-of-the-art is about 50GHz. Thus, we can generate high-purity stable 200GHz photonic local signals by using this technique, which would be useful for large scale millimeter wave antenna arrays.