This is a re-release of the final version of the 4th Edition of Plan 9 from Bell Labs distributed directly by Bell Labs.

4th Edition was originally released by Lucent Technologies in 2002. It was released under the terms the Lucent Public License Version 1.02. 4th Edition continues the development of the same ideas, but in many ways represents the base of what most people think of as "modern" Plan 9 today. The ubiquitous file protocol, 9P, got its first revision, to 9P2000, which caused changes to many parts of the system. It also saw the debut of venti(8) and fossil(4), which began replacing the venerable old file server, fs(4). Authentication largely moved to a separate agent, factotum(4). Updates moved to replica, and the wiki was created. The original release notes for this edition can be found at /sys/doc/release4.ms.

While this release is a recognizable ancestor of the latest Plan 9 available, it is still quite old, and this release is mostly for educational and research purposes. The file protocol and authentication mechanisms are continued in modern Plan 9, though, so researchers looking to run this should have an easier time than with all previous releases. This is a second release of the 4th Edition, marking the final state of Plan 9 as distributed directly by Bell Labs.

The only changes between this release and the original 4th Edition release are the new NOTICE, LICENSE, and README files, and moving the old ones. In particular, any copyright notices attributing things to Lucent Technologies or Bell Labs, or notices on restrictions of their use, should be read as historical; all such rights have been transferred to the Plan 9 Foundation and released as open source. Parts (notably, the fonts) maintain trademark and copyright to their respective parties (such as Bigelow & Holmes Inc.) and were included in Plan 9 by permission; those copyrights are unaffected by the transfer to Plan 9 Foundation and remain in effect per the terms found in this distribution.