EXT4 is a next generation file system replacement for the EXT2/EXT3 family of Linux file systems. It was accepted as "stable" in the Linux 2.6.28 kernel in October 2008. As of this writing, it's starting to appear as the default file system in newer versions of several Linux distros. While the developers did try to maintain some degree of backwards compatibility with EXT2/EXT3, there is quite a bit that's new and different with EXT4. Popular forensic tools like the Sleuthkit are not fully compatible with these changes in EXT4, although some of their functionality does still work.