HTML table with single cell at bottom

HTML table with single cell at bottom

I got a html table which looks basically like this
(http://jsfiddle.net/LMaQq/):
<table>
<thead>
<tr>
<th>Col 1</th>
<th>Col 2</th>
<th>Col 3</th>
<th>Col 4</th>
</tr>
</thead>
<tbody>
<tr>
<td>Content</td>
<td>Content</td>
<td>Content</td>
<td><input type="checkbox"></td>
</tr>
<tr>
<td>Content</td>
<td>Content</td>
<td>Content</td>
<td><input type="checkbox"></td>
</tr>
</tbody>
</table>
What I want to do now is to add a new table cell only under Col 4. Under
Col 1 to Col 3 there shouldn't be anything. So it should look like this:
http://pl.vc/4fg4v (Sorry, I can't upload the image directly here, because
10 reputation is needed for this)
The background for this is, that I want another checkbox under the
existing ones, which allows me to select/deselect all of them at the same
time. How can I achieve this?