Is there way to bypass passcode in iPhone 4? IOS7?

My phone was stolen the other day and they turned my phone off so I can't use Find my iPhone. I'm just worried that once they turn it back in, they find a way to bypass my passcode and look through my files. I have credit card details in my phone so I'm really worried.

my 3G isn't turned on so even if they turn my phone on, the only way i can delete all my data using "Find my iPhone" is if they connect to Wi-Fi.

Remote wipe it if you have find my phone turned on. No one can get your info until it is on anyway and only if they have your apple id password. That is the security of ios7.