User groups

Hi guys,
I have a question about user roles. I have a group of authors that post to the Blog. It is a company blog, covering 4 different departments. What I would like to do is make one person from each department an editor, being able to edit other people´s posts – however only people from their department.
I even downloaded some Plugins, but was not able to find a solution to this, because when I make someone an editor, they can edit anything they want. My goal is for them to only be able to edit posts of authors from their department and not authors or editors from other departments.
Is this possible in WP or via some plugin? It seems to me like a relatively common situation, which should be solvable.

Thanks in advance for any help