int pin[] = {15, 4, 5, 18};

int ledState = LOW;
unsigned long previousMillis = 0;
long interval = 1000;

int ledState1 = LOW;
unsigned long previousMillis1 = 0;
long interval1 = 1000;

int ledState2 = LOW;
unsigned long previousMillis2 = 0;
long interval2 = 1000;

int ledState3 = LOW;
unsigned long previousMillis3 = 0;
long interval3 = 1000;

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

void loop() {
  /*
    for (int i = 0; i <= 3; i++) {
    digitalWrite(pin[i], HIGH);
    digitalWrite(pin[i], 0);
    delay(1000);
    }
  */


  unsigned long currentMillis = millis();

  if (currentMillis - previousMillis >= interval) {
    // save the last time you blinked the LED
    previousMillis = currentMillis;

    // if the LED is off turn it on and vice-versa:
    if (ledState == LOW) {
      ledState = HIGH;
      interval = 300;
    } else {
      ledState = LOW;
      interval = 1500;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(15, ledState);
  }

  unsigned long currentMillis1 = millis();

  if (currentMillis1 - previousMillis1 >= interval1) {
    // save the last time you blinked the LED
    previousMillis1 = currentMillis1;

    // if the LED is off turn it on and vice-versa:
    if (ledState1 == LOW) {
      ledState1 = HIGH;
      interval1 = 1500;
    } else {
      ledState1 = LOW;
      interval1 = 300;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(4, ledState1);

  }
  unsigned long currentMillis2 = millis();

  if (currentMillis2 - previousMillis2 >= interval2) {
    // save the last time you blinked the LED
    previousMillis2 = currentMillis2;

    // if the LED is off turn it on and vice-versa:
    if (ledState2 == LOW) {
      ledState2 = HIGH;
    } else {
      ledState2 = LOW;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(5, ledState2);


  }
  unsigned long currentMillis3 = millis();
  if (currentMillis3 - previousMillis3 >= interval3) {
    // save the last time you blinked the LED
    previousMillis3 = currentMillis3;

    // if the LED is off turn it on and vice-versa:
    if (ledState3 == LOW) {
      ledState3 = HIGH;
    } else {
      ledState3 = LOW;
    }

    // set the LED with the ledState of the variable:
    digitalWrite(18, ledState3);


  }
}