Linus Torvalds has integrated a large collection of patches prepared by (Ext) filesystem developer Theodore Ts’o (tytso) into the main development branch of Linux. It contains a patch for Ext4 which allows it to present itself as ext4 instead of ext4dev. This signals that with the next kernel version 2.6.28, the successor to ext3 will finally leave behind its “hot” development phase. The kernel developers integrated an early version of Ext4 in the main development branch of Linux 2.6.19 in order to jointly develop it to maturity there.
This by no means concludes the development of Ext4 and it is expected that the developers will steadily continue to develop the filesystem in the same way they developed Ext3, not only fixing bugs but also adding the occasional function. The filesystem structures used on storage media are no longer going to be affected by these changes – therefore, the file backup and media reformatting tasks required during the development of Ext4 within the official Linux kernel should no longer be necessary. Users will also no longer be required to ensure that their kernel version, Ext4 utilities (e2fsprogs) and storage media formats match up.
This email by Ts’o sent to the Linux Kernel Mailing List a while back tries to position ext4 as a bridge to btrfs.