Can the person who stolen my iPhone break the passcode?

My iPhone 5s as been stolen it as a passcode on and iv reported it stolen so it's been blacklisted the problem is I'm really worried about whoever as taken my phone being able to see very personal stuff on ther pics texts between me and my boyfriend is ther anyway a 4 digit pass code can be undone by any phone expert? I'm sick with worry iCloud as never been set up on phone and I don't know my Apple ID for that phone please help

Yes, if somebody really wanted to crack the passcode, it's not that hard to crack a 4-digit code. However, many phones do have a feature whereby if you enter 10 incorrect codes the phone just locks up forever, so if your phone had that feature you're almost certainly safe.

They most likely won't be able to break the passcode, but because you don't have Icloud set up i'm pretty sure they will just reset your phone and will be able to use it but everything on it will be deleted so they won't see it.