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.
IRC
From Reiser4 FS Wiki
There is an IRC channel for reiser4 and reiserfs discussion on OFTC:
#reiser4 on irc.oftc.net
