void setup() {
  for (int i = 4; i <= 13; i++) {
    pinMode(i, OUTPUT);
  }
}

void loop() {
  for (int i = 4; i <= 13; i++) {
    digitalWrite(i, HIGH);
    delay(100);
    digitalWrite(i, LOW);
  }
  for (int i = 12; i > 4; i--) {
    digitalWrite(i, HIGH);
    delay(100);
    digitalWrite(i, LOW);
  }
}