Steps to create and mount ext4 filesystem

Syntax: 

mkfs.ext4  "Device Name"

(or)

mke2fs -t ext4 "Device Name"

Example:

[root@hostxyz bin]# mkfs.ext4 /dev/sda3

mke4fs 1.41.9 (22-Aug-2009)

Filesystem label=

OS type: Linux

Block size=4096 (log=2)

Fragment size=4096 (log=2)

67108864 inodes, 268435456 blocks

13421772 blocks (5.00%) reserved for the super user

First data block=0

Maximum filesystem blocks=4294967296

8192 block groups

32768 blocks per group, 32768 fragments per group

8192 inodes per group

Superblock backups stored on blocks:

        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,

        4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,

        102400000, 214990848

Writing inode tables: done

Creating journal (32768 blocks): done

Writing superblocks and filesystem accounting information: done

[root@hostxyz ~]# mkdir /ext4_filesystem

[root@hostxyz ~]# mount -t ext4 /dev/sda3  /ext4_filesystem


[root@hostxyz ~]# file -sL /dev/sda3
/dev/sda3: Linux rev 1.0 ext4 filesystem data (needs journal recovery) (extents) (large files) (huge files)
[root@hostxyz ~]#

Note: Ext4 filesystem is available starting from Kernel 2.6.19. Incase your server don't have the "ext4" package installed, you can avail it by installing the latest version of "e4fsprogs" rpm.
(URL to download for CentOS: http://pkgs.org/package/e4fsprogs ).
However it is recommended to use the Ext4 filesystem from the kernel version mentioned above to get the full advantage of it.