Is Jailbreaking And Enabling Bluetooth File Transfer On iPhone 4 Financially Secure?
Will jailbreaking, downloading unofficial apps (not registered in Apple's App Store) and enabling Bluetooth file transfer cause a chance of sensitive data theft? 


There's a chance, but if you download from verified sources and check the repository you're going to be downloading from (google searches, etc.) you should be okay.  There's always a risk, but as with any risk, if you're vigilant about things you'll be fine.


  Is sim free iPhone already jailbroken?


If you're talking about an iPhone without the sim card, no, that doesn't mean that the phone is jailbroken already.  It just means it's the phone without the sim card.  iPhones have removable sim cards in them.