#include <FastLED.h>

#define NUM_LEDS  100

CRGB led1[NUM_LEDS];
CRGB led2[NUM_LEDS];
CRGB led3[NUM_LEDS];
CRGB led4[NUM_LEDS];

int time_gap = 500;

int r = 255;
int b = 0;
int g = 0;

void setup() {
  FastLED.addLeds<WS2812B, 9, GRB>(led1, NUM_LEDS);
  FastLED.addLeds<WS2812B, 10, GRB>(led2, NUM_LEDS);
  FastLED.addLeds<WS2812B, 11, GRB>(led3, NUM_LEDS);
  FastLED.addLeds<WS2812B, 12, GRB>(led4, NUM_LEDS);
  FastLED.setBrightness(100);
}

int prevMillis = millis();
int currMillis = millis();

int i = 0, j = 0, k = 0, l = 0;

void loop() {
  i = 0, j = 0, k = 0, l = 0;

  while (l <= NUM_LEDS - 1) {
    if (i < NUM_LEDS) led1[i++] = CRGB(r, g, b);
    if (j < NUM_LEDS && currMillis - prevMillis >= time_gap)led2[j++] = CRGB(r, b, g);
    if (k < NUM_LEDS && currMillis - prevMillis >= time_gap * 2)led3[k++] = CRGB(r, b, g);
    if (l < NUM_LEDS && currMillis - prevMillis >= time_gap * 3) led4[l++] = CRGB(r, b, g);
    FastLED.show();
    delay(30);
    currMillis = millis();
  }

  prevMillis = millis();

  while (l > 0) {
    if (i > 0)led1[--i] = CRGB(0, 0, 0);
    if (j > 0 && currMillis - prevMillis >= time_gap)led2[--j] = CRGB(0, 0, 0);
    if (k > 0 && currMillis - prevMillis >= time_gap * 2)led3[--k] = CRGB(0, 0, 0);
    if (l > 0 && currMillis - prevMillis >= time_gap * 3)led4[--l] = CRGB(0, 0, 0);
    FastLED.show();
    delay(30);
    currMillis = millis();
  }

  FastLED.clear();
  prevMillis = millis();
}