const int LED_PINS[] = {5,18,19,21};
const int NUM_LEDS   = sizeof(LED_PINS)/sizeof(int);

#define OFF (HIGH) // active-low LED

void setup() {
  for ( int i=0; i < NUM_LEDS; i++ ) {
    // set the direction of the i-th LED pin
    pinMode( LED_PINS[i], OUTPUT ); 
    digitalWrite( LED_PINS[i], OFF );
  }
}

const int PWM_RESOLUTION = 8;
const int PWM_FREQ = 1000;
const int DUTY_MAX = (1 << PWM_RESOLUTION);

void loop() {
  for ( int i=0; i < NUM_LEDS; i++ ) {
      ledcSetup( i /*channel*/, PWM_FREQ, PWM_RESOLUTION );
      ledcAttachPin( LED_PINS[i] /*pin*/, i /*channel*/ );
      for ( int x=0; x <= DUTY_MAX; x++ ) {
         ledcWrite( i, DUTY_MAX-x );
         delay(1);
      }
  }
  for ( int i=0; i < NUM_LEDS; i++ ) {
      ledcSetup( i /*channel*/, PWM_FREQ, PWM_RESOLUTION );
      ledcAttachPin( LED_PINS[i] /*pin*/, i /*channel*/ );
      for ( int x=0; x <= DUTY_MAX; x++ ) {
         ledcWrite( i, x );
         delay(1);
      }
      ledcDetachPin( LED_PINS[i] ); 
      digitalWrite( LED_PINS[i], OFF );
  }
}