TexasWebDevelopers--A Dallas, Texas Web Design and Development Company.

This goes in between the <head> and </head> tags: 
        <script language="JavaScript" type="text/JavaScript">
        <!--
        var checked = 0;
        function addCheck(box)
        {
        // allow checked box to be unchecked
        if(!box.checked) return true;
        // get ref to collection // see Alt:
        var boxes = document.getElementsByName(box.name);
        // count checked
        var cb, count=0, k=0;
        while(cb=boxes[k++])
          if(cb.checked && ++count>4){
          alert("Sorry, you can only check 4 boxes.");
          return false; 
          }
          return true;
          }
           // Alt: Use this line
           // - to support non-DOM browsers (v4)
           // - only used if boxes are in a form
           // var boxes = box.form.elements[box.name];
           //-->
        </script>
		
This is the code for the checkboxes: 
<input type="checkbox" name="category[]" onClick="return addCheck(this);">