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Last year EXT4 was announced as stable release and it was merged in kernel 2.6.28 as part of the coder afterwards, it brings many great features am stating some here before getting on how to move an existing partition to ext4 without formatting

Large file system
The ext4 filesystem can support volumes with sizes up to 1 Exbibyte (1,152,921,504,606,846,976 bytes) and files with sizes up to 16 tebibytes (1,099,511,627,776 bytes).

Extents
Extents are introduced to replace the traditional block mapping scheme used by ext2/3 filesystems. An extent is a range of contiguous physical blocks, improving large file performance and reducing fragmentation. A single extent in ext4 can map up to 128MB of contiguous space with a 4KB block size. There can be 4 extents stored in the inode. When there are more than 4 extents to a file, the rest of the extents are indexed in an Htree.

Persistent pre-allocation
The ext4 filesystem allows for pre-allocation of on-disk space for a file. The current methodology for this on most file systems is to write the file full of 0s to reserve the space when the file is created. This method would no longer be required for ext4; instead, a new fallocate() system call was added to the Linux kernel for use by filesystems, including ext4 and XFS, that have this capability. The space allocated for files such as these would be guaranteed and would likely be contiguous. This has applications for media streaming and databases.

my favorite ;)
Journal checksumming
Ext4 uses checksums in the journal to improve reliability, since the journal is one of the most used files of the disk. This feature has a side benefit; it can safely avoid a disk I/O wait during the journaling process, improving performance slightly.

NOTE
Filesystem changes/hacks contains certain risk ,though we tested this on our system we don’t guarantee this will work perfectly on your systems and we shall not be responsible for any data loss or hard disk crash .
p.s. backup your system before proceeding .

Step 1 :
Assuming we are changing sda2 edit its line in /etc/fstab to look like this

/dev/sda2     /                ext4    defaults      1  1

Step 2 :
Make sure needed packages are up-to-date then update your initrd image

yum upgrade mkinitrd e2fsprogs
mv /boot/initrd-`uname -r`.img /boot/initrd-`uname -r`.img.old
mkinitrd -v --with=ext4 /boot/initrd-`uname -r`.img `uname -r`

Step 3 :
Boot using your OS CD-ROM into rescue mode and run this commands to apply the new FS extensions , make sure you’re working on the correct partition and you’re using

/mnt/sysimage/sbin/tune2fs -I 256 -O has_journal,extents,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize /dev/sda2

The last command should tell your to run fsck and reboot if it didn’t it has failed , and you should remove the tune2fs option it asks to be removed

Last step :

fsck -pf /dev/sda2