Arduino UNO CNC shield - controlling 4 stepper motors

Using Arduino LEDs and Multiplexing
4

You use the 4th motor (A) just like the other 3 except the step pin for the A is pin 12 and the dir pin is pin 13 once the jumpers are in place. That means that you no longer have the spindle enable or spindle direction control.

#define CW 0
#define CCW 1

const byte stepPin = 12; // x axis > 2, y axis > 3, z axis > 4 a axis > 12
const byte dirPin = 13; // x axis > 5, y axis > 6, z axis > 7 a axis > 13
const byte enablePin = 8; // enable on the CNC shield is held HIGH (disabled) by default

unsigned long stepTime = 2400;

void setup()
{
   Serial.begin(115200);
   pinMode(stepPin, OUTPUT);
   pinMode(dirPin, OUTPUT); 
   pinMode(enablePin, OUTPUT);
   digitalWrite(enablePin, LOW);  // enable steppers
   digitalWrite(dirPin, CCW);
}

void loop()
{
  oneStep();
}

void oneStep()
{
   static unsigned long timer = 0;
   unsigned long interval = stepTime;
   if (micros() - timer >= interval)
   {
      timer = micros();
      digitalWrite(stepPin, HIGH);
      delayMicroseconds(10);
      digitalWrite(stepPin, LOW);
   }
}

If you need a limit (home) switch for the A axis (as well as x, y, z), you will need to use one of the analog pins for the sensor.