add a class after 4 itrations each time through jquery

[Knuxer]

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\[code\] <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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+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="http://stackoverflow.com/questions/14039642/#">+ZOOM</a></span> <br clear="all"> </div> </div>\[/code\]here is the script:\[code\] $('div.impresna-wrapp').find('div.impresna:nth-child(5)').each(function() { $(this).addClass("alter-impresna"); });\[/code\]