Ext4 pushed into Kernel 2.6.19
The fourth implementation of ext file system is pushed into 2.6.19 development tree.
Ext4 is backward compatible with Ext3, so systems without Ext4 support can mount Ext4 partitions.
The main new feature of Ext4 is the support of Extents:
An extent is a contiguous area of storage in a computer file system, reserved for a file. When starting to write to a file, a whole extent is allocated. When writing to the file again, possibly after doing other write operations, the data continues where the previous write left off. This reduces or eliminates file fragmentation.
From wikipedia
The Extents are disabled for default, to guarantee the backward compatibility.
Read more here!