If you are using maven you can just use a previous version number as they should all be in the FUSE maven repo

http://repo.fusesource.com/

If you want to download versions with FUSE installers etc. I actually don't know.

Ah the installers is also in the repo:

eg here is for FUSE ESB 4.0.0 released last year:

http://repo.fusesource.com/maven2/com/iona/fuse/fuse-esb/4.0.0.0/

You can click on parent directory and choose another version eg 4.0.0.3