Trying to get a Bootstrap Carousal to show 4 items fixed

I found this:

css:

* { box-sizing: border-box; }

body { font-family: sans-serif; }

.scene {
  border: 1px solid #CCC;
  margin: 40px 0;
  position: relative;
  width: 210px;
  height: 140px;
  margin: 40px auto;
  perspective: 1000px;
}

.carousel {
  width: 100%;
  height: 100%;
  position: absolute;
  transform: translateZ(-288px);
  transform-style: preserve-3d;
  transition: transform 1s;
}

.carousel__cell {
  position: absolute;
  width: 190px;
  height: 120px;
  left: 10px;
  top: 10px;
  border: 2px solid black;
  line-height: 116px;
  font-size: 80px;
  font-weight: bold;
  color: white;
  text-align: center;
}

.carousel__cell:nth-child(9n+1) { background: hsla(  0, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+2) { background: hsla( 40, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+3) { background: hsla( 80, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+4) { background: hsla(120, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+5) { background: hsla(160, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+6) { background: hsla(200, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+7) { background: hsla(240, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+8) { background: hsla(280, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+0) { background: hsla(320, 100%, 50%, 0.8); }

.carousel__cell:nth-child(1) { transform: rotateY(  0deg) translateZ(288px); }
.carousel__cell:nth-child(2) { transform: rotateY( 40deg) translateZ(288px); }
.carousel__cell:nth-child(3) { transform: rotateY( 80deg) translateZ(288px); }
.carousel__cell:nth-child(4) { transform: rotateY(120deg) translateZ(288px); }
.carousel__cell:nth-child(5) { transform: rotateY(160deg) translateZ(288px); }
.carousel__cell:nth-child(6) { transform: rotateY(200deg) translateZ(288px); }
.carousel__cell:nth-child(7) { transform: rotateY(240deg) translateZ(288px); }
.carousel__cell:nth-child(8) { transform: rotateY(280deg) translateZ(288px); }
.carousel__cell:nth-child(9) { transform: rotateY(320deg) translateZ(288px); }

html:

<div class="scene">
<div class="carousel">
<div class="carousel__cell">1</div>
<div class="carousel__cell">2</div>
<div class="carousel__cell">3</div>
<div class="carousel__cell">4</div>
<div class="carousel__cell">5</div>
<div class="carousel__cell">6</div>
<div class="carousel__cell">7</div>
<div class="carousel__cell">8</div>
<div class="carousel__cell">9</div>

javascript:

var carousel = document.querySelector('.carousel');
var cellCount = 9;
var selectedIndex = 0;

function rotateCarousel() {
  var angle = selectedIndex / cellCount * -360;
  carousel.style.transform = 'translateZ(-288px) rotateY(' + angle + 'deg)';
}

  var prevButton = document.querySelector('.previous-button');
 prevButton.addEventListener( 'click', function() {
    selectedIndex--;
 rotateCarousel();
  });

 var nextButton = document.querySelector('.next-button');
  nextButton.addEventListener( 'click', function() {
   selectedIndex++;
  rotateCarousel();
 });

 https://codepen.io/desandro/pen/jxwELK

But cant seem to get 4 fixed items to work, any help, i stripped some parts of the css and javascript but didn't display the items correctly.

I tried this css:

.carousel__cell:nth-child(9n+1) { background: hsla(  0, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+2) { background: hsla( 40, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+3) { background: hsla( 80, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+4) { background: hsla(120, 100%, 50%, 0.8); }
.carousel__cell:nth-child(9n+5) { background: hsla(160, 100%, 50%, 0.8); }

  .carousel__cell:nth-child(1) { transform: rotateY(  0deg) translateZ(128px); 
}
.carousel__cell:nth-child(2) { transform: rotateY( 90deg) translateZ(128px); 
}
.carousel__cell:nth-child(3) { transform: rotateY( 180deg) translateZ(128px); 
}
.carousel__cell:nth-child(4) { transform: rotateY(360deg) translateZ(128px); 
}
.carousel__cell:nth-child(5) { transform: rotateY(160deg) translateZ(288px); 
}

and change the cell count to 4.

But im missing a cell