const int LED_PINS[] = {21, 19, 18, 5, 4, 2, 15, 22};
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);
const float DUTY_CYCLES[] = {0.1,0.25,0.5,1};

void loop() {
  for ( int i=0; i < NUM_LEDS; i++) {
    for ( int j=0; j < 4; j++) {
      ledcSetup( j, PWM_FREQ, PWM_RESOLUTION );
      ledcAttachPin( LED_PINS[(i+j)%NUM_LEDS], j );
      ledcWrite( j, (1 - DUTY_CYCLES[j])*DUTY_MAX );
    }
    delay(100);
    for ( int j=0; j < 4; j++) {
      ledcDetachPin( LED_PINS[(i+j)%NUM_LEDS] );
      digitalWrite( LED_PINS[(i+j)%NUM_LEDS], OFF );
    }
  }
}