Most of the documentation is a snapshot of the old Namesys site (archive.org, 2007-09-29).
There was also a Reiser4 Wiki (archive.org, 2007-07-06) once on pub.namesys.com.
Reiser4 development appears to have stalled and ReiserFS is deprecated and scheduled to be removed from mainline Linux in 2025.
Reiserfsprogs
From Reiser4 FS Wiki
The tools to maintain a ReiserFS (Reiser v3) filesystem are called reiserfsprogs and can be found on the openSUSE Build Service. If your distribution does not ship a pre-compiled package, we have to build this manually:
sudo apt-get install uuid-dev # Debian, Ubuntu sudo yum install libuuid-devel # Fedora sudo zypper install libuuid-devel # openSUSE wget https://ftp.kernel.org/pub/linux/kernel/people/jeffm/reiserfsprogs/v3.6.21/reiserfsprogs-3.6.21.tar.{sign,xz} xz -d reiserfsprogs-3.6.21.tar.xz gpg --verify reiserfsprogs-3.6.21.tar.sign tar -xf reiserfsprogs-3.6.21.tar cd reiserfsprogs-3.6.21 ./configure --prefix=/opt/reiserfsprogs make && sudo make install