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.
Filesystem Testing Tools
From Reiser4 FS Wiki
stress.sh
A more general filesystem testing tool really, not specific to ReiserFS or Reiser4 is stress.sh, by Chris Mason.