Installing Plesk 8.x on debian 4.x “etch” seems a bit uncommon, but the mixture of a rockstable and secure base system as debian and the common webserver management interface Plesk makes sense.

How to install Plesk on debian

1. Install a fresh debian and perform a dist-upgrade:
   apt-get update && apt-get dist-upgrade
2. Add the following line to the apt sources list located at /etc/apt/sources.list (replace 8.2.1 and etch with your versions)
   deb http://autoinstall.plesk.com/debian/PSA_8.2.1 etch all
3. Force apt-get to update database with new information from Plesk
   apt-get update
4. Install Plesk base system with apt-get, say Yes to accept the packages from Plesk that have a problematic signature.
   apt-get install psa
5. Install Plesk Autoupdate module with apt-get:
   apt-get install psa-autoinstaller
6. Install package with PHP5 suport for Plesk
   apt-get install psa-php5-configurator
7. Restart PSA – don’t forget this! Otherwise you’ll get 404 errors instead the login page.
   /etc/init.d/psa restart
8. Access the fresh installed Plesk interface with your browser (replace your-server-name with the url/ip of your server). The default login is admin, the default password setup.
   https://your-server-name:8443/
9. That’s it. The rest can be done via the Plesk interface.

More information can be found in the Plesk v8.2.1 build82070918.10 for Debian-4.0 x86_64 Release Notes.