Wordpress Themes - WP Forum at BFA - changing post title to h4

I want to change the post title (on multiple post pages) from h2 to h4 (or h1, doesn't really matter as long as it's not h2 or h3).

right now the html for the first post headline (On my website: www.opengreenitpolicy.org) looks like this:

HTML Code:

<div class="post-headline">                <h2>                        

                        <a href="http://www.opengreenitpolicy.org/?p=62" rel="bookmark" title="Permanent Link to Green IT Conference 2010">Green IT Conference 2010</a></h2>

                </div>

I want to change the <h2> into <h4> but I have no clue where to do this. Anybody know how to do this?

Thanks!
Ivo