Ext4 is mandatory for now

Ext4 is the default filesystem for newly created virtual servers (mandatory) now. It doesn't affect already created virtual servers with ext3.
It is modern filesystem much more faster than ext3. Is stable and reliable. Supported both by RHEL5 and RHEL6.
e4fsprogs is missing

Package e4fsprogs is required by ext4 support for RHEL5. On RHEL6 this dependency is provided by e2fsprogs
Ext4 tools fixes for RHEL5

On RHEL5 the userspace tools are called e4fsck, resize4fs or dumpe4fs