Do you really need that many fans? I suggest you remove at least one. I would remove 1 front fan and put the remain three to blow into the case. The 2 rear and 1 top should blow out.
If all fans run the same speed, one additional fan wouldn't do much good. If it blows in, it would blow more air in than there comes out. Usually this gives various wind noises from the fans where you try to push air through.
<i><b>Engineering is the fine art of making what you want from things you can get</b></i>
Front fans pulling in. Rear and top throwing the air out.
Here's <A HREF="http://www.amd.com/us-en/assets/content_type/white_pape..." target="_new">one article, with diagrams</A>, if you wanna look. Check the diagram on page labeled 16, or hit "Chassis Airflow" on the left side. It doesn't actually show a fan on the top, but you can see that it would go with the theme.