:nth-last-of-type

/* Selects every fourth <div> inside a parent element */
/* Counting backwards from the last one */
div:nth-last-of-type(4n) {
  background-color: lime;
}

:nth-last-of-type( <nth> )where 
<nth> = <an-plus-b> | even | odd

<div>
  <span>This is a span.</span>
  <span>This is another span.</span>
  <em>This is emphasized.</em>
  <span>Wow, this span gets limed!!!</span>
  <strike>This is struck through.</strike>
  <span>Here is one last span.</span>
</div>

span:nth-last-of-type(2) {
  background-color: lime;
}