int ledPins[] = {2, 3, 4, 5}; // an array of four digital output pins for the LEDs
int currentDigit = 0; // index of the current hex digit being displayed

void setup() {
  // set the digital pins as output
  for (int i = 0; i < 4; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // display the current hex digit on the LEDs
  switch (currentDigit) {
    case 0:
      digitalWrite(ledPins[0], HIGH);
      digitalWrite(ledPins[1], LOW);
      digitalWrite(ledPins[2], LOW);
      digitalWrite(ledPins[3], LOW);
      break;
    case 1:
      digitalWrite(ledPins[0], LOW);
      digitalWrite(ledPins[1], HIGH);
      digitalWrite(ledPins[2], LOW);
      digitalWrite(ledPins[3], LOW);
      break;
    case 2:
      digitalWrite(ledPins[0], LOW);
      digitalWrite(ledPins[1], LOW);
      digitalWrite(ledPins[2], HIGH);
      digitalWrite(ledPins[3], LOW);
      break;
    case 3:
      digitalWrite(ledPins[0], LOW);
      digitalWrite(ledPins[1], LOW);
      digitalWrite(ledPins[2], LOW);
      digitalWrite(ledPins[3], HIGH);
      break;
  }
  
  // advance to the next hex digit
  currentDigit = (currentDigit + 1) % 4;
  
  // wait for a short period of time before displaying the next digit
  delay(500);
}