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.
Reiser4 patchsets
stable patchsets
As reiser4 is still not in mainline, Edward Shishkin is kind enough to provide patches for the stable version of the Linux kernel.
development patchsets
Andrew Morton is maintaining the MMOTM tree (mm-of-the-moment) that includes Reiser4 as well. Snapshot patches from that tree can be found on kernel.org. Be aware that this tree in high flux and often filled with exotic stuff (yes, more exotic than reiser4 ;-))
After applying the patches please test and report any issues!