:nth-of-type

/* Selects every fourth <div> inside a parent element */
div:nth-of-type(4n) {
? background-color: lime;
}

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

/* Align odd paragraphs to the left */
p:nth-of-type(2n+1) { text-align: left; }

/* Align even paragraphs to the right */
p:nth-of-type(2n) { text-align: right; }

<div>
??? <div>This element isn't counted.</div>
??? <p>First paragraph. (left aligned)</p>
??? <p>Second paragraph. (right aligned)</p>
??? <div>This element isn't counted.</div>
??? <p>Third paragraph. (left aligned)</p>
</div>