Si usando una tabella.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.hidden {display:none}
.shown  {display:block}
.mainMenu {color:white;background-color:navy;font-size:9pt;font-family:verdana;cursor:hand}
.subMenu {color:navy;background-color:white;font-size:9pt;font-family:verdana;cursor:hand}
-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function showHide(aDiv) {
 aDiv.className = aDiv.className == "hidden" ? "shown" : "hidden";
} // function showHide(aDiv)
//-->
</script>

</head>
<body>
<table >
<tr>
 <td valign="top">
  <div style="width:100px" class="mainMenu" onclick="showHide(document.getElementById('subMenu1'))">Menu1</div>
  <div id="subMenu1" class="hidden">
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 1</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 2</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 3</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 4</div>
  </div>
 </td>
 <td valign="top">
  <div style="width:100px" class="mainMenu" onclick="showHide(document.getElementById('subMenu2'))">Menu2</div>
    <div id="subMenu2" class="hidden">
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 1</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 2</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 3</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 4</div>
  </div>
 </td>
</tr>
<tr>
 <td valign="top">
  <div style="width:100px" class="mainMenu" onclick="showHide(document.getElementById('subMenu3'))">Menu3</div>
  <div id="subMenu3" class="hidden">
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 1</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 2</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 3</div>
   <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 4</div>
  </div>
 </td>
 <td valign="top">
  <div style="width:100px" class="mainMenu" onclick="showHide(document.getElementById('subMenu4'))">Menu4</div>
    <div id="subMenu4" class="hidden">
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 1</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 2</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 3</div>
    <div style="width:100px" class="subMenu" onmouseover="this.style.backgroundColor='yellow'" onmouseout="this.style.backgroundColor='white'"> Voce 4</div>
  </div>
 </td>
</tr>
</table>
</body>
</html>