In Mac OS X Tiger Apple implemented Access Control Lists (ACL's) in order to provide finer grained control over file and folder permissions. 4D obeys these ACL's along with POSIX privileges.
For more information on ACLs, please visit:
ACL's are not enabled by default on OS X; you must enable ACLs for each drive you wish to utilize them on. To do this on OS X Server, you can utilize the Workgroup Manager or the following command will enable them on the boot volume:
% sudo /usr/sbin/fsaclctl -p / -e
Once ACL's are enabled the Workgroup Manager allows graphical editing of ACL settings. The chmod command can add or remove ACE's and insert them in specific positions in the ACL. Remember, it is very significant that ACLs are ordered lists; the first rule that matches, "wins".