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 should be shipped by most distributions. If it doesn't, we have to build it manually.
Install prerequisites:
sudo apt-get install uuid-dev libacl1-dev comerr-dev # Debian, Ubuntu sudo dnf install libuuid-devel libacl-devel libcom_err-devel # Fedora sudo zypper install libuuid-devel libacl-devel libcom_err-devel # openSUSE
Get source:
VER=3.6.25 wget https://www.kernel.org/pub/linux/kernel/people/jeffm/reiserfsprogs/v$VER/reiserfsprogs-$VER.tar.{sign,xz} xz -d reiserfsprogs-$VER.tar.xz gpg --recv-keys 2179E5B2 gpg --verify reiserfsprogs-$VER.tar.sign tar -xf reiserfsprogs-$VER.tar && cd reiserfsprogs-$VER
Build & install:
./configure --prefix=/opt/reiserfsprogs && make # Prefix with CFLAGS="$CFLAGS -std=gnu89" for GCC-5 sudo make install
Or, from the Git tree:
git clone git://git.kernel.org/pub/scm/linux/kernel/git/jeffm/reiserfsprogs.git reiserfsprogs-git cd reiserfsprogs-git libtoolize --copy --install --force && aclocal && autoheader && autoconf && automake --add-missing ./configure --prefix=/opt/reiserfsprogs && make sudo make install