A melhor resposta provavelmente é do próprio ext4 Wiki :
With recent versions of ext4 (2.6.29 and later), you can mount any ext2 or ext3 filesystem as ext4 without any changes. You must use tune2fs to enable the new ext4 features:
# tune2fs -O extents,uninit_bg,huge_file /dev/DEV # e2fsck -f /dev/DEV
Once you have enabled extents a former ext2 or ext3 filesystem, it is an ext4 filesystem and cannot be reverted to the old format.
Mas note que
Some ext4 features cannot be enabled on an existing ext3 filesystem.
e que você deve ver o Ext4 Howto para obter mais detalhes.