JSFiddle - React, Tailwind, and code Playground

HTML

<div class="promo">
    <div class="container">


        <div class="row">
            <div class="col-xs-12 col-sm-6 col-md-6 col-lg-3">
                <div class="panel panel-default">
                    <div class="panel-heading panel-heading-custom">NEW OPENING</div>
                        <div class="panel-body">
                            <img src="images/img.gif" class="img-circle center-block margins" />
                            <p>To celebrate our new opening, we have sales on many of your favorite items.</p>
                        </div>
                </div>
            </div>

            <div class="col-xs-12 col-sm-6 col-md-6 col-lg-3">
                <div class="panel panel-default">
                    <div class="panel-heading panel-heading-custom">QUALITY ASSURRED</div>
                        <div class="panel-body">
                            <img src="images/img2.gif" class="img-circle center-block margins" />
                            <p>Our products come with a five-year warranty.</p>
                        </div>
                </div>
            </div>

            <div class="col-xs-12 col-sm-6 col-md-6 col-lg-3">
                <div class="panel panel-default" id="tallest">
                    <div class="panel-heading panel-heading-custom">IN-STORE SPECIAL</div>
                        <div class="panel-body">
                            <img src="images/img3.gif" class="img-circle center-block margins" />
                            <p>Take advantage of our new in-store special. You won't be disappointed to find the greatest deals. Now when you buy two or more products, you can get a third product of similar value for free. </p>
                        </div>
                </div>
            </div>

            <div class="col-xs-12 col-sm-6 col-md-6 col-lg-3">
                <div class="panel panel-default">
                    <div class="panel-heading panel-heading-custom">FREE CONSULTATIONS</div>
              ...

CSS

.panel {
  background:red;
  border-bottom:1px solid blue;
}

JavaScript

var maxH = 0;

$('.panel').each(function(){
if($(this).height() > maxH) {
  maxH = $(this).height();
}
});

$('.panel').height(maxH);