int led[]={0,1,2,3};
void setup() {
  // put your setup code here, to run once:

  for (int i=0;i<=3;i++){
    pinMode(led[i], OUTPUT);
  }
}

void loop() {
   for (int i=0;i<=3;i++){
    digitalWrite(led[i], HIGH);
    delay(500);
    digitalWrite(led[i], LOW);
    delay(500);
  }
}