Jean,
I should have been clearer in my explanation. This isn't a user feature in 4.0.0 that allows users to delete any backup.
What I was trying to say was that 4.0.0 includes the ability to delete any backup, while maintaining the consistency of the reverse deltas. That simplifies the deletion logic. In contrast, 3.x doesn't allow a backup to be deleted that others depend on for the deltas. It's not a user-level feature though.
Craig