water cooled hotend - the Kraken

Would the SD3 handle the extra weight?

This would be cool!

I'm not sure I would ever use 4 nozzles but it is definitely awesome!

Yes, I would assume 4 separate steppers. Making anything else work might get complicated.

How would you control 7 steppers?

Rumba can handle 3 extruders, but it seems they stopped too soon.

given the physical arrangement means simultaneous printing options will be limited, there is always the perspective that you potentially dont need multiple drivers - just a 'safe' (for the drivers - meaning, set the motor to 'off', which disables the stepper chip) way to toggle which stepper is connected active to the stepper.

You potentially could do it all of a standard sanguinololu with just a small carrier board...

Send M84 E to disable the Extruder until the next extruder move, and thus sticking the stepper driver in sleep mode (well, disables it via its enable pin anyway).

As for getting it to switch the appropriate mux (i'm imagining this would be best done as digitally switched small ssr's driving some *FETs.... ) ; then right now, would be easiest to run it through a post-process filter in RH that simply looks for the various snippets of tool-change gcode and does a simple 'insert line with M84 E ; M42 P7 S255; M42 P8 S0; M42 P9 S255' ; where P7, P8 and P9 are the mythical pins I have chosen to be the inputs to the mux to switch the input states to the steppers motor header and 0 or 255 control their state. The next extruder move will auto-re-enable the stepper, so no need to worry about that part. Would be easy enough to do this in python, perl, php or whatever your preferred script poison is. Meh, slic3r via its custom g-code might have some ability to add extra tool-change code anyway, in which case, it can be all done sans script....

Anyway - thats one way you could skin the cat.

yeah, shift register, mux, its only the surronding circuitry that gets tweaked and the software codes - but same same outcome.

And yeah, if you google around, adrian bowyer i believe had a modified marlin to send gcode to a second board to drive multiple extruders as part of his colour mixing experiments....

Impressive design... but you would still have four separate colors. And afraid by the weight of 4 stepper motors. The hotend itself is quite light.
I think I saw a printer with 4 feeders and one extruder, with the ability to generate intermediate colors by mixing them when they get melted in the same extruder. Don't know how you manage to manage the feeders to take into account the various percentages of colors required.

I presume you could just use three of the extruders and drive it with the rumba board?

the azteeg x3 with the latest shield can support 4 hotends and a heated bed as well as up to 8 steppers   Although I beleive most firmwares only support up to 3 hotends...

how much does the extra hot ends affect the build area?