Sim card for Iphone 4s/5

So basically, i really want an iPhone 5, if not iPhone 4s but i have a contract that is still got 1 year left, so basically i was wondering if i got my cousins old iPhone 4s i could put my sim from my current phone (blackberry) in it and still have my contract without the blackberry on the iPhone 4s? Or would i need to get a new sim or a completely new iPhone? I'm very confused at what to do! Please don't say wait until your contract is up as i can't, i hate my phone, it never holds its charge or anything! My mum is willing to pay some if i pay some as well.

Thank you, all help is welcome!

Added (1). Can i transfer the things on my current sim (including my monthly contract) onto a sim for an iphone?

Most cell phones are "Locked" to the wireless carrier that they were made for. If your cousin and you are on the same carrier than it is a possibility.

Also the SIM card size might be different, they come in different sizes.

1) Check to see what carrier your cousin's phone was on when they got it.
2) If they are not the same, then the iPhone would have to be unlocked, generally by the original carrier.
3) Check to see if the SIM cards for both phones are the same size / dimensions.
4) Personally I would also call your wireless carrier, hopefully at a retail location, and explain what you'd like to do, and see if they can do that.