A few tipps if you are forced to do that upgrade, for example because you are using a hosting provider like server4you that didnt update their virtual server images for over two years.

Basically you patch up your debian 4 to the latest version using the debian archive servers, add the archive server pubkeys, then update plesk from 8.4 to 8.6, to 9.2.2, then to 9.2.3, then to the latest version. After that you update debian 4 to 5 by replacing the etch entries in your sources (/etc/apt/…) and calling apt-get update/upgrade.

If you need to patch up your debian 4 you use the following repositories:

# etch packages.
deb http://archive.debian.org/debian/ etch main contrib non-free
deb http://security.debian.org/ etch/updates main contrib non-free

# source packages.
deb-src http://archive.debian.org/debian/ etch main contrib non-free
deb-src http://security.debian.org/ etch/updates main contrib non-free

If you get the W: GPG error: http://security.debian.org etch/updates Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 9AA38DCD55BE302B error then do the following:

apt-get install debian-archive-keyring
apt-get update