<ul class="flex-container">
  <li class="flex-item">1</li>
  <li class="flex-item">2</li>
  <li class="flex-item">3</li>
  <li class="flex-item">4</li>
</ul>
.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
  display: flex;
    
  -webkit-justify-content: space-around;
  justify-content: space-around;
  -webkit-flex-flow: row wrap;
  flex-flow: row wrap;
  -webkit-align-items: stretch;
  align-items: stretch;
}

.flex-item:nth-of-type(1) { flex-grow: 1; }
.flex-item:nth-of-type(2) { flex-grow: 1; }
.flex-item:nth-of-type(3) { flex-grow: 2; }
.flex-item:nth-of-type(4) { flex-grow: 1; }
.flex-item:nth-of-type(5) { flex-grow: 1; }

.flex-item {
  
  background: tomato;
  border: 3px solid rgba(0,0,0,.2);
  line-height: 100px;
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js