void setup() {

  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);

  pinMode(A0, INPUT);

  Serial.begin(9600);

}

void loop() 
{
  static unsigned long lastTime = 0;
  static unsigned char phase = 0;

  unsigned long now = millis();

  int period = analogRead(A0);
  period = map(period, 0, 1023, 83, 750);

  if (now - lastTime < period) return;
  lastTime = now;

//  Serial.println(analogRead(A0));

  switch (phase & 0x3) {    // just use bottom two bits of the phase counter
  case 0 :
    digitalWrite(3, LOW);   
    digitalWrite(4, LOW);    
    break;

  case 1 :
    digitalWrite(3, LOW);   
    digitalWrite(4, HIGH);    
    break;

  case 2 :
    digitalWrite(3, HIGH);   
    digitalWrite(4, HIGH);    
    break;

  case 3 :
    digitalWrite(3, HIGH);   
    digitalWrite(4, LOW);    
    break;
  }

  phase++;
}