"Chris Samuel: ext4 is a much bigger change from ext3 than ext3 was in turn from ext2. Ext3 essentially just added journalling to ext2, whereas ext4 moves to an extent based filesystem with other features such as delayed allocations (like XFS) to allow the allocator to be more intelligent about how it lays things out
on disk and much bigger filesystem sizes (though the programs to create ext4 filesystems can't actually make them for you yet). Another nice feature is the fact that the journal data is checksummed so the filesystem can spot any corruption after a crash."
"So the upsides are all the new features, but the downside is that there is an awful lot of new code here and while it has been in the kernel for a while marked 'experimental' it's not had the years of testing and hardening that ext3 has gone through."
