you should use jquery to remove your target4 element from your page

1<script>
2  $(document).ready(function() {
3    $("#target1").css("color", "red");
4    $("#target1").prop("disabled", true);
5    $("#target4").remove();
6  });
7</script>
8
9<!-- Only change code above this line. -->
10
11<div class="container-fluid">
12  <h3 class="text-primary text-center">jQuery Playground</h3>
13  <div class="row">
14    <div class="col-xs-6">
15      <h4>#left-well</h4>
16      <div class="well" id="left-well">
17        <button class="btn btn-default target" id="target1">#target1</button>
18        <button class="btn btn-default target" id="target2">#target2</button>
19        <button class="btn btn-default target" id="target3">#target3</button>
20      </div>
21    </div>
22    <div class="col-xs-6">
23      <h4>#right-well</h4>
24      <div class="well" id="right-well">
25        <button class="btn btn-default target" id="target4">#target4</button>
26        <button class="btn btn-default target" id="target5">#target5</button>
27        <button class="btn btn-default target" id="target6">#target6</button>
28      </div>
29    </div>
30  </div>
31</div>