Can i put my blackberry contract SIM card in an Iphone 4s?

I have recently been given a iphone4s (16gb white) and i want to put my blackberry contract SIM card in it. I have over a year left on my contract and i don't want to cancel it. Can i just i put my contract SIM in the iphone or do i have to get a new contract?

Added (1). If i have to get my SIM cut down will my network provider do that or will i have to?

Yes.

Is the iphone a sim free one? Otherwise youll have to jailbreak it

You would have to cut the SIM to microSIM size 1st. Also if the iPhone is not on the same network as the BlackBerry you would need to get it unlocked as well.

Yes, as long as the iPhone is locked to your network or unlocked. You will need to either cut down your sim to micro-sim size, or contact your network to send you out a micro sim.