You can already do this with the forum.
An administrator can set individual permissions per member, giving them access to specific forums.
It can be done via the Admin area, once there you want the Member Permissions link. You can see it on a demo here:
https://www.webwiz.net/web-wiz-forums/forum-demo.htmIt might be difficult to manage with 4000 members, but then I doubt an average forum requires member specific permissions for that many members.
I do have a mod that will list out what members have [url=http://www.s2h.co.uk/wwf/mods/permissions-listing/user specific permissions here[/url], but it wasn't designed for a large usage like yours. Also it is currently out of date, it is only a link in the admin area that would likely need updating on it though.