Ubuntu 9.04 does not use ext4 by default
There seems to be a significant misconception around Ubuntu 9.04, that it uses the (comparatively new) ext4 filesystem by default. It doesn’t. Ubuntu 9.04 still uses the tried-and-true ext3 filesystem by default. If you install Ubuntu 9.04 and use automatic partitioning, you will get ext3, which we consider to be the most stable option at this time.
ext4 is available as an option in the manual partitioning screen for people who wish to try out the latest stuff. There are some known bugs which affect this configuration, though, and filesystem bugs generally carry the risk of lost data, so this is not for the faint of heart.
A Linux Weekly News article included an error (now corrected) in this regard, and even our own Ubuntu Weekly Newsletter reprinted a BeginLinux.com article which implies that 9.04 carries the risks of ext4.
We included the manual ext4 option for people who want to help test this new technology, to help its developers isolate and fix any remaining bugs so that it can become the new standard for Linux and the default for Ubuntu. Many people are interested in it, have opted to try it, and written about the experience (which is probably how the misconception started).
In short, don’t panic. Ubuntu 9.04 isn’t any more likely to eat your data than any previous version.