I just downloaded the Zimbra 4.0 Open Source Edition and looked through the docs that came with it for upgrade instructions. The only thing I found was a note in the Zimbra_Release_Note.pdf that said that upgrade instructions could be found by going to http://www.zimbra.com/community/ and clicking on Documentation. I did that and I'm not seeing anything. Is that something that is going to be up soon?
btw, the link in the pdf is broken. The url includes the word after the link so it actually took me to http://www.zimbra.com/community/and