On a new install of both CentOS 5.8 and 1.600-1 Webmin there is no option to format a logical volume with ext4. 1.5 Webmin and 1.5 upgraded to 1.6 Webmin on CentOS 5.8 have this option. RH says ext4 is supported.
Does your system have the mkfs.ext4 command installed? If that is missing, Webmin will not present the option to create an EXT4 filesystem.
You are entirely correct - the package was missing. Thanks for your assistance!