Well, turns out, I wasn't. They just skipped the number 4 all together and jumped straight to 5, so that it can use the same version numbers as Firefox and it'll also be on the rapid development schedule.
So we can look forward to Firefox and Thunderbird 9.0 before Christmas.

http://download.cnet.com/8301-2007_4-20075955-12/thunderbird-joins-firefox-with-rapid-release/


