JQuery UI Sortable - Need Help

JQuery UI Sortable - Need Help

Hello, I m new to JQuery. My problem is like this. I have 3 UL element, one of the UL element have 8 LI element that can be drag n drop on to any of the other 2 UL element. I can do it by setting the code below
      $("ul.droptrue").sortable({
         connectWith: 'ul'
      });

      $("#sortable1, #sortable2, #sortable3").disableSelection();

my problem is that i want to limit the LI element that the other 2 UL element can accept. each of the other 2 UL element can only accept 4 LI element. when the UL element have 4 LI element, it cannot accept any LI element from the 1st UL element. Can anyone teach me how to do this? Thanks. In other word i want to control the size of the UL element.