const int ledPins[] = {2, 3, 4, 5};
const int numLeds = 4;

unsigned long previousMillis = 0;
const unsigned long interval = 100; // time each LED stays on (ms)

int currentLed = 0;

void setup() {
  for (int i = 0; i < numLeds; i++) {
    pinMode(ledPins[i], OUTPUT);
    digitalWrite(ledPins[i], LOW);
  }
}

void loop() {
  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;

    // Turn off all LEDs
    for (int i = 0; i < numLeds; i++) {
      digitalWrite(ledPins[i], LOW);
    }

    // Turn on the current LED
    digitalWrite(ledPins[currentLed], HIGH);

    // Move to next LED
    currentLed++;
    if (currentLed >= numLeds) {
      currentLed = 0;
    }
  }
}