In my last post, I was uncertain as to what some of the I/O plugs were for on the RAMPS. Fortunately, I was able to find it pretty quickly on the Reprap wiki:
so E0 / E1 are for the extruder motors, D10 is for the extruder heater, and D08 is for the heated bed.
I still need to find out what Aux4 is for.
update: Aux 4 is for a as-yet-unsupported LCD screen. The SFJW firmware apparently supports it, but it’s very alpha at the moment. (It doesn’t even support 200k thermistors!)