I agree with matttail that you should check with Support and please let us know what you learn. On my IMAP Service Providers page in the DreamHost section, I say that a DreamHost IMAP INBOX is limited to 3000 messages, but I don't know if that is still true. My strategy is to file all my messages in submailboxes (aka subfolders), i.e., IMAP boxes that are under the INBOX. This also seems to hide messages from the daemon that likes to move messages to the .old-messages box. As far as I know, you can use your entire 4800+ MB for IMAP-accessible messages (as long as they aren't in INBOXes!).
Hope this helps,
Infinite Ink ~ http://www.ii.com
Deflexion & Reflexion ~ http://deflexion.com