If you're an OS X user then one thing need to be aware of is these two releases bump the minimum OS X version to 10.6 (Snow Leopard). The Linux builds (x86 and x64) are built on Ubuntu 11.04.
A lot of work went into 0.4.0. New features and bug fixes galore. See the ChangeLog for full details. I have to thank Strahinja Marković (the original creator) for leaving the 0.4.0 in a nearly finished state. Also, Charles King for being a bug fixing monster and helping make this release great.
One major change that is going to take place next week is, I am switching the source code management (SCM) system from Mercurial (hg) to Git. This will break the update checker in previous versions. Hence waiting a week before making the change. I have already converted FlightCrew (needed to test) so FlightCrew's updater in previous version won't inform you of an update. This is unfortunate but unavoidable.
Due to the change to Git the links for the manual in Sigil are not going to work for a week. This is because I've updated the locations in Sigil to point to the new locations that won't be available until the switch to Git.