2 Pioneer CDJs and a DJM - Making use of all 4 channels, and routing effects to a different channel
Just a nifty little finding that I hadn't seen in any posts here.

When I got my DJM 800 I was kind of bummed that I couldn't route the fx to a different channel to EQ the fx or do anything funky with. The DJM itself has a ton of fx, but that lack of control was a real bummer for me. I was watching some ellaskins videos where he uses the digital output AND the line output from his CDJs to do some funky wacky thing that ellaskins does, and I realized this would be a perfect way to duplicate audio from a CDJ to the mixer, allowing me use 2 channels as master channels, and 2 channels specifically for fx (where I could use the EQ, color fx and master fx to screw with the sound). It's a great way to make use of all 4 channels on the mixer, AND effectively route fx to different channels.

TL;DR - Plug the Line out AND Digital out from your CDJs to 2 different channels on your 4 channel mixer (same track on 2 channels) and use one of those channels specifically for FX. Edit: (think send/return)