I got this puzzle in an e-mail, the answer of which is the password to a file attached (it is safe). The answer I get is 9618, but it can't open the file. Am I doing something wrong?

Scenario:

There is a bus with 7 girls inside,

Each girl has 7 bags,

Inside each bag, there are 7 Big cats,

Each Big cat has 7 small cats,

All cats have 4 Legs each!

Question: How many Legs are there inside the bus?