Can you put a sim in a sprint iphone4?

I'm going to buy an iphone from someone and it's a great price but it is only for sprint. I'm a student on a budget so I'm on the straight talk plan. I researched and it says I can put a Sim in an at&t iphone if unlocked/jailbroken. I don't know if I can do the same for sprint though. Can I replace a straight talk Sim in a sprint phone like an at&t iphone?

You can do that only if your device is unlocked otherwise you can't.

You will have to unlock your phone first and then you will be able to do so
try this link for unlocking your iphone
http://www.vunlock.com/