Ext4

ext4 is an open source disk filesystem and the most recent version of the ext series.

Kernel
You need to activate the following kernel option:

If you have large disks (2 TB or larger), and are not using a 64-bit kernel, also enable the following option:

Software
contains the utilities to work with the filesystem. It's part of the system set and so already installed on your system.

You can change the USE flags:

After setting this you want to update your system so the changes take effect:

Creation
To create an ext4 filesystem on the partition:

Please replace with the actual partition you want to format.

By default, 5% of available disk space is reserved for the root user. This is usually a good thing for the partition where the directory is mounted, but it may be not desiderable on other partitions. If you don't want to reserve disk space for the root user you can use the  option:

Mount
You can mount the filesystems in several ways:
 * mount - Manual mount.
 * fstab - Automatic mount at boot time.
 * removable media - Automatic mount at demand.
 * AutoFS - Automatic mount on access.

External resources

 * ext4 wiki