// Arduino UNO - 4 LEDs ON one by one

int leds[] = {2, 3, 4, 5};  // LED pins

void setup() {
  // Configure pins as outputs
  for (int i = 0; i < 4; i++) {
    pinMode(leds[i], OUTPUT);
  }
}

void loop() {
  // Turn LEDs ON one by one
  for (int i = 0; i < 4; i++) {
    digitalWrite(leds[i], HIGH);  // Turn ON LED
    delay(500);                   // Wait 0.5 second
    digitalWrite(leds[i], LOW);   // Turn OFF LED
  }
}