Linux-Plesk 9 in RHEL 4 : Webmail shows default page.

Try this:

Download and install the following RPM

 http://autoinstall.plesk.com/PSA9/dist-rpm-RedHat-el4-i386/9.0.1/opt/horde/

Then execute - 

/usr/local/psa/admin/bin/webmailmng --install --name=horde

/usr/local/psa/admin/sbin/websrvmng -a -v

Hope it fixes the issue..