int i = 0;
const int LED1 = 3; //define the pin we use for LED
const int LED2 = 6;
const int LED3 = 9;
const int LED4 = 11;

void setup() {
  pinMode(LED1, OUTPUT); //set pin 3 as OUTPUT
  pinMode(LED2, OUTPUT); //set pin 6 as OUTPUT
  pinMode(LED3, OUTPUT); //set pin 9 as OUTPUT
  pinMode(LED4, OUTPUT); //set pin 11 as OUTPUT
}

void loop() {
    for (int i = 0; i < 255; i++){ //if i is less than 255 then increase i with 1
    analogWrite(LED1, i); //write the i value to pin 3
    analogWrite(LED2, i); //write the i value to pin 6
    analogWrite(LED3, i); //write the i value to pin 9
    analogWrite(LED4, i); //write the i value to pin 11
    
    delay(5); //wait 5 ms then do the for loop again
  }
  for (int i = 255; i > 0; i--){ //descrease i with 1
    analogWrite(LED1, i);
    analogWrite(LED2, i);
    analogWrite(LED3, i);
    analogWrite(LED4, i);
    delay(5);
  }
}