[Kirk Y]

<if condition="in_array($GLOBALS[forumid], 5,6,7,8">
<img src="/path/to/header_$GLOBALS[forumid].gif" />
<else />
<img src="/path/to/global_header.gif" />
</if>

So what you need to do is rename the 4 forum-specific headers you have to "header_X", where X is the Forum ID where each header should be displayed.

Replace 5,6,7,8 with the Forum IDs that you'd like the specific headers displayed in.