I'm very tempted by the E3D Kraken hot end, which has four extruders in one hot end. At this point it's very much a DIY technology, not used in any "off the shelf" printer. And there are relatively few controllers that can drive four extruders. So I'm curious if anyone using Simplify3D has tried it yet.
i got it running with duel just fine on a Rmax, my concern is although it says you have have t0-t3 = 4 extruders in the process, t2 is actually my bed temp not t2 extruder so that could be an issue. i would just say it has to be implemented and pointed the right way and the rest would be up to the firmware you are running. but with a heated bed it looks like you would be limited to only 3 extruders