But...
in line with what Juno and Sixkitten say, it has now become EXTREMELY hard to get an extension after 4 years. I have not known anyone receive a fail for going over. However, I do know people who have been disadvantaged by the new rules, including a close friend who has a chronic illness that has previously necessited her getting extensions to her registration. Her department is now trying to bully her into suspending her studies, and she is very upset by the sudden change in treatment of her case.
To me, it makes no sense to automatically fail someone for going over 4 years, especially if they are motivated and have good reasons for needing more time. It looks bad for the uni if nothing else. While it is important to get students through in a timely manner, it's also important to acknoweldge that PhDs AREN'T straightforward entities that can be contained like taught courses.