In the ~8 years since the 4th edition release, there has been pretty continuous work on Plan 9, both at Bell Labs and in the 9fans community; nightly an ISO is constructed and uploaded. Changes have been incremental -- a tool appears, bugs are fixed, etc..
http://acm.jhu.edu/git/plan9 has a git tree of the sources from December 2002 through February 2009, if you like to view changes in that form. http://bitbucket.org/rminnich/sysfromiso has a mercurial tree of more recent changes, if that's what you're looking for. -- vs
