My iPhone 4 just got stolen today, can they use it still?

I had it on Unlock with no password. I didn't realise my phone falling out of my pocket when getting out from my car so whoever found it, made a stupid facebook status update and clocked my phone. (He probably used his simcard next) Will he still have access to my personal things? I changed my facebook and email password. I didn't have the app Find my iphone on it so i won't know where it is.

I called the service provider and asked them to lock my phone and they said they will lock the sim and to lock the phone it will take 4hrs or so. So if the phone is locked, will they STILL be able to use it?

Maybe if they change the sim or factory reset the phone via iTunes

As you've reported it as lost/stolen your network provider will block you SIM so the person who has it can't use your credit or run you up a bill. Your network will replace your SIM free of charge with the same number.

The will also block the phones IMEI number (its serial number). This makes the phone useless and it will not be able to be used with any SIM card. Once a phone is barred like this it can't be unlocked/unbarred unless YOU as the owner called your network and asked them to unbar it.

Unfortunately any info on the phone such as text messages, photos etc will still be able to be seen by the person who has it if the phone is on - but once the person who has it turns it off or the battery goes - when they try and turn it on again it will come up that the phone is barred so therefore they won't be able to get into your personal stuff again.