The ext4, or fourth extended filesystem is a journalled file system that was revealed on October 10, 2006 by Andrew Morton as a compatible improvement to the ext3, featuring support for volumes up to 1 exabyte (1024 petabytes) and added extent support.
The ext4 filesystem is backward compatible with ext3, that is, it can be mounted as an ext3 partition (using “ext3” as the filesystem type when mounting). Similarly, mounting an ext3 filesystem as ext4 is also possible (using the “ext4dev” filesystem type). However, if the ext4 partition uses extents (one of the major new features of ext4), backward compatibility and therefore the ability to mount the filesystem as ext3 is lost. Extents are not used by default; the “extents” option is explicitly required (e.g.
mount /dev/sda1 /mnt/point -t ext4dev -o extents).