If i found an iPhone 4 can i use it?

Last night i went to watch a movie and i found an iPhone 4. I was wondering if i can use it with my number if its the same carrier. I will need a micro SIM card so will AT&T blame me for stealing an iPhone? So can I use it?

That would be theft.

It would be theft but you won't be caught by AT&T so yes you can use it.

I would honestly call somebody from contact list to see who the phone belongs to. I would feel terrible if somebody kept my phone. But that's just my opinion.

if this phone company starts asking you questions just tell them that you lost all your papers for the phone. And if the cops come knocking at the door flush the phone and deny everything.

You can switch the carrier if it happens to be a different one (just take it to the at&t store.
it is impossible to have 2 phones with the same number but if you wanted you can get call forwarding which means if you don't answer the iPhone it would directly call your other one. If you meant that you wanted to switch your number to that, it can also be done (by them). Just to be sure to crack the phones password or lock before you give it to them, that way there's no chance of a confrontation as to why you don't know it. It can usually done by looking how to do it on YouTube.