Webform radio button sets common functionality
AFAIK javascript is the only option and I do not see any help in the Drupal webform module 

You could do it effectively in javascript/Jquery.

Another option as Beebee suggested you could force all the 4 radio buttons into a single set.

&lt;div id="content"&gt;
 &lt;input type=radio name="chooseme" value="1" /&gt;
 &lt;input type=radio name="chooseme" value="2"/&gt;
 &lt;input type=radio name="chooseme" value="3"/&gt;
 &lt;input type=radio name="chooseme" value="4"/&gt;
&lt;/div&gt;


Now,user would be left to choose any one of the four radios.