My Nested Boxes

HTML

<div class="big_box">BOX1</div>
<div class="big_box">BOX2</div>
<div class="big_box">
  <div class="small_box">BOX3</div>
  <div class="small_box">BOX4</div>
</div>

CSS

.big_box{
  height: 8em;
  width: 8em;
  border: 1px solid black;
  float: left;
}
.small_box{
  height: 4em;
  width: 8em;
  border: 1px solid black;
}