[Issvar]

SELECT postid FROM post LEFT JOIN thread ON thread.threadid=post.threadid WHERE thread.forumid IN (xx,xy,xz) AND thread.dateline=post.dateline ORDER BY post.dateline DESC LIMIT 4

That's the mysql statement you probably want. It gets the postids from the first posts of the latest threads in the forums with forumid xx,xy and xz (change as required). Avatars are accessed by their postid so you should be able to use them now easily. You can add getting the post content to that mysql statement so you can automize that as well. To do so just replace the SELECT postid with SELECT postid, pagetext