Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<style type="text/css">
ul#container_4 { list-style-type: none; width: 415px; margin: auto; }
ul#container_4 li { float: left; display: block; background: #000; width: 100px; height: 100px; margin-right: 5px; margin-bottom: 5px; }
ul#container_4 li.omega { margin-right: 0px; }
</style>
</head>
<body>
<ul id="container_4">
<li></li>
<li></li>
<li></li>
<li class="omega"></li>
<li></li>
<li></li>
<li></li>
<li class="omega"></li>
<li></li>
<li></li>
<li></li>
<li class="omega"></li>
<li></li>
<li></li>
<li></li>
<li class="omega"></li>
</ul>
</body>
</html>