I've been working with linux for a little while now. All the books/tutorials I've read say that file/directory permissions can be summed up with a 3 number combo: a number for owner, group and other. So I understand this part of the permissions.
Once article I've read was specifying permissions in the 4 number combo: 0775
What is the fourth number? 3 of them must be owner,group,other but what is the fourth?