add a class after 4 itrations each time through jquery

Question

what i need to do is add a class on 1st 5th 9th 13th div and so on but the script that i wrote is hardcoded which work fine for me but the problem is that it add class to 5th one only but i need 1, 5, 9 and so on

  <div class="impresna-wrapp">
                        <div class="impresna ">
            <p><img src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-     1.jpg" alt="impresna-img-1" width="203" height="101" class="alignnone size-full wp-image-46"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>
                    <div class="impresna ">
            <p><img class="alignnone size-full wp-image-46" alt="impresna-img-1" src="http://localhost/AON/wp-content/uploads/2012/12/impresna-img-1.jpg" width="203" height="101"></p>
            <p><span class="emboss">EXTRA</span> | Ronaldo arremata capacete de Rubens Barrichello em leil�o de caridade em torneio de golf...</p>
            <a href="#">+MAIS</a>
            <br clear="all">
        </div>


                            <div class="impresna  zoom-img" id="zoom-img">
            <p><img src="http://localhost/AON/wp-content/uploads/2012/12/impresna-zoom-img.jpg" alt="impresna-zoom-img" width="122" height="170" class="alignnone size-full wp-image-66"></p>
            <span class="main"><span class="emboss">MARCA - SP</span> 20.09.12
            <a href="#">+ZOOM</a></span>
            <br clear="all">
        </div>
                    <div class="impresna  zoom-img" id="zoom-img">
            <p><img src="http://localhost/AON/wp-content/uploads/2012/12/impresna-zoom-img.jpg" alt="impresna-zoom-img" width="122" height="170" class="alignnone size-full wp-image-66"></p>
            <span class="main"><span class="emboss">MARCA - SP</span> 20.09.12
            <a href="#">+ZOOM</a></span>
            <br clear="all">
        </div>

    </div>

here is the script:

           $('div.impresna-wrapp').find('div.impresna:nth-child(5)').each(function() {
           $(this).addClass("alter-impresna");
            });

Answer 1

You need to use nth-child(4n+1)

Live Demo

$('div.impresna-wrapp').find('div.impresna:nth-child(4n+1)').each(function() {
           $(this).addClass("alter-impresna");
});