the iso on the bell labs site is built nightly so if you have that you have the up-to-date source.
I wouldn't expect there will be a fifth edition as most of the plan9 team from bell labs now work at google. Having said this it might happen, it depends on the priorities inside lucent-alcatel. you call update from a mounted iso image if you have no network but i cannot remember the exact runes, maybe the 9fans archives will help? the third edition is not very interesting, it is very similar to the fourth but has a different 9p protocol (and a few other things). the seccond was purchased like a book for $270 (from memory), the cdroms do appear (with license card one hopes) on ebay from time to time. I archived the four distribution floppies of the binary only "demo" release for posterity. the first edition was given to universities who expressed an interest, like Unix was in its early days. the license for ed1 was restrictive so these cdroms don't appear - some people might still have the single demo floppy that Rob gave out in london but thats your only hope. I put some historic papers and distribution images (copyright permitting) on http://plan9.bell-labs.com/sources/contrib/steve/historic, and I believe uriel has more on http://www.cat-v.org -Steve
