const int LED1 = 2;   // LED connected to digital pin 2
const int LED2 = 3;   // LED connected to digital pin 3
const int LED3 = 4;   // LED connected to digital pin 4
const int LED4 = 5;   // LED connected to digital pin 5

void setup() {
  // Set the four LED pins as outputs
  pinMode(LED1, OUTPUT);
  pinMode(LED2, OUTPUT);
  pinMode(LED3, OUTPUT);
  pinMode(LED4, OUTPUT);
}

void loop() {
  // Turn on LED1 and LED3
  digitalWrite(LED1, HIGH);
  digitalWrite(LED3, HIGH);
  // Turn off LED2 and LED4
  digitalWrite(LED2, LOW);
  digitalWrite(LED4, LOW);
  // Wait for 500 ms
  delay(500);

  // Turn on LED2 and LED4
  digitalWrite(LED2, HIGH);
  digitalWrite(LED4, HIGH);
  // Turn off LED1 and LED3
  digitalWrite(LED1, LOW);
  digitalWrite(LED3, LOW);
  // Wait for 500 ms
  delay(500);
}