JSFiddle - React, Tailwind, and code Playground

HTML

<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>
<div class="thumbnail"></div>

JavaScript

$('.thumbnail:nth-child(4n)').each(function(){
   $(this).prevAll('.thumbnail').andSelf().wrapAll('<div class="new"></div>') 
});