unsigned long startTime1 = 0;
unsigned long startTime2 = 0;
unsigned long startTime3 = 0;
unsigned long startTime4 = 0;

int ledState1 = LOW;
int ledState2 = LOW;
int ledState3 = LOW;
int ledState4 = LOW;

void setup() {

pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
pinMode(10, OUTPUT);
}

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

if (currentTime - startTime1 > 1000){
  startTime1 = millis();

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

    // set the LED with the ledState of the variable:
    digitalWrite(13, ledState1);
}


if (currentTime - startTime2 > 2000){
  startTime2  = millis();
     // if the LED is off turn it on and vice-versa:
    if (ledState2 == LOW) {
      ledState2 = HIGH;
    } else {
      ledState2 = LOW;
    }

  digitalWrite(12, ledState2);
}


if (currentTime -startTime3 > 3000){
  startTime3  = millis();
     // if the LED is off turn it on and vice-versa:
    if (ledState3 == LOW) {
      ledState3 = HIGH;
    } else {
      ledState3 = LOW;
    }

  digitalWrite(11, ledState3);
}

if (currentTime - startTime4 > 4000){
  startTime4  = millis();
     // if the LED is off turn it on and vice-versa:
    if (ledState4 == LOW) {
      ledState4 = HIGH;
    } else {
      ledState4 = LOW;
    }

  digitalWrite(10, ledState4);
}

}