How to get the id of previous ul

I have a div with 4 seperate lists, I need to know the id of the list before the one i click on.

So lets say i click on ul2 i need an alert of ul1 id.

currently im getting the clicked id with:

  var clicked_id = this.id;

and ive tried the following to get the above ul

  var div_id2 = $(this).parents(".divtest").next(".ultest").attr("id");
  var div_id = $(this).closest('ul').find('.ultest').attr('id');
  var x = $(this).prev().attr('id');

simplification of layout:

  <div id="doesnt matter" class="divtest">
    <ul id="ul1" class="ultest">
      <li id="doesnt matter"></li>
      <li></li>
      <li></li>
    </ul>
    <ul id="ul2" class="ultest">
      <li></li>
      <li></li>
      <li></li>
    </ul>
    <ul id="ul3" class="ultest">
      <li></li>
      <li></li>
      <li></li>
    </ul>
    <ul id="ul4" class="ultest">
      <li></li>
      <li></li>
      <li></li>
    </ul>
  </div>