//EX 2.1 SHORT CODE
#define LED1 1
#define LED2 5
#define LED3 9
#define LED4 13

int leds[] = {LED1, LED2, LED3, LED4};

void setup() {
}

void loop() {
  for (int i = 0; i < 16; i++) {
    for (int j = 0; j < 4; j++) {
      pinMode(leds[j], OUTPUT);
    }
    for (int j = 0; j < 4; j++) {
      digitalWrite(leds[j], i & (1 << j) ? HIGH : LOW);
    }
    delay(1000);
  }
}