tune4fs can use many undocumented (I mean they are not yet in man tune4fs) options for ext4 filesystem. For example, huge_file, flex_bg, dir_nlink, extra_isize and some others. Is there a comprehensive list of them with description what each option does?
I would say that you should look in
/src/linux-2.6/Documentation/filesystems/ext4.txt
but those options don't seem to be there in the current pull. So I would search the ext4 wiki, and if you don't find it there, search and/or post on the ext4 mailing list.