Put a stepper on the 4th axis that connects to the "knob" on your speed controller. TurboCNC supports this style of speed controlling, as does Mach.
The "DigiSpeed" is the electronic equivalent of the same, a virtual stepper driving a digital potentiometer.
On/off is usually done with a relay, which you can slap together for about $10 in parts.
http://www.dakeng.com/relay.html