Edit in JSFiddle

<div class="container">
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
</div>

.container {
  width: 100%;
  background: #fff;
}
.container:after {
  display: block;
  width: 0;
  height: 0;
  content: '';
  clear:both;
}

.element {
    width: 20%;
    background-color: green;
    float: left;
    margin-right: 10px;
    position: relative;
}

.element:before {
    display: block;
    content: "";
    padding-top: 75%; /* 4:3 */
}