Just talking to Tanker about the attempted bombings in London today, and I got to thinking, last week they used 4 bombs In London

, Sept 11 there were 4 planes

, and I can't remember the details about Madrid

.
Why fours? Is there any significance?
I know there have been other attacks (I'm thinking Abu Bakar that fuck) in Australia and Trinidad where they didn't bomb in fours.
Is four the number of terror?