// constants won't change. They're used here to
// set pin numbers:
const int led1 = 12;
const int led2 = 11;
const int led3 = 10;
const int led4 = 9;

// Variables will change:
int ledState = LOW;             // ledState used to set the LED

// Generally, you should use "unsigned long" for variables that hold time
// The value will quickly become too large for an int to store
unsigned long previousMillis = 0;        // will store last time LED was updated

// constants won't change:
const long interval = 500;           // interval at which to blink (milliseconds)

void setup() {
  // set the digital pin as output:
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);
  pinMode(led4, OUTPUT);
}

void loop()
{
  // check to see if it's time to change the state of the LED
  unsigned long currentMillis = millis();

  if(currentMillis - previousMillis > interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW)
      ledState = HIGH;
    else
      ledState = LOW;

    // set the LED with the ledState of the variable:
    digitalWrite(led1, ledState);
    delay(500);
    digitalWrite(led2, ledState);
    delay(500);
    digitalWrite(led3, ledState);
    delay(500);
    digitalWrite(led4, ledState);
  }
}