JSFiddle - React, Tailwind, and code Playground

HTML

<div id="postings">
    <div class="contents">1</div>
    <div class="contents">2</div>
    <div class="contents">3</div>
    <div class="contents">4</div>
    <div class="contents">5</div>
    <div class="contents">6</div>
    <div class="contents">7</div>
    <div class="contents">8</div>
    <div class="contents">9</div>
    <div class="contents">10</div>
    <div class="contents">11</div>
    <div class="contents">12</div>
</div>

CSS

.new
{
    width:100%; 
    border:1px solid; 
    margin:10px; 
}

JavaScript

var divs = $(".contents");
for(var i = 0; i < divs.length; i+=4) {
   j = i + 1; 
  divs.slice(i, i+4).wrapAll("<div id='newDivforWraping"+j+"' class='new'></div>");
}