Even if file permissions are set on a document, other domain administrators will still have full access to that document. The same goes for Group Policy Objects (GPO) in Active Directory. GPOs can be set to restrict access to objects, like documents, to a fairly high degree of granularity. But, again, that won't stop a domain administrator.
There are a few workarounds. One option might be something old-fashioned: put confidential documents on a separate network or on a workstation. Another possibility is to create a separate group for the other three administrators that doesn't have full administrative rights. These users would have to use either "sudo" for Linux or "runas" for Windows. These commands restrict administrative access for particular users for particular functions.
Again, for only four domain administrators, your options are limited, and the best course might be just to keep your confidential documents off the network on an isolated workstation.
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This was first published in October 2007