Wow, and 4,5 bitcoins are a huge amount of money, they are more than 45 thousand dollars, sure that it is a huge amount of money, how were they stolen?
The best thing that you can do is just nothing, because there is no way to contact blockchain.info support because they are not going to answer to you, and if they do, they are not going to give you any details about that person because it is illegal. Just think about it, why if i say that you are the scammer and i want to ask blockchain for your IP and they gave it to me.. it would be strange.
Because of these thiefs,innocent people are even considers thiefs.So very insane