Another ways to reduce FWM effect are the increment of the effective area of your optical fiber, which, in its turn will decrease the non-linear coefficient or increase the spacing between the spectral channels (if it is applicable in your case). I would also like you to take a look at one of the Optiwave examples that describes the properties of FWM: https://optiwave.com/resources/applications-resources/optical-system-effects-of-cross-phase-modulation-xpm-and-four-wave-mixing-fwm/.