What will happen if I mail apple my cracked iPhone 4 for repair?

My iPhone 4 screen is cracked. If I mail it into Apple, will they just mail me back a whole new iPhone 4, or will it actually still be my old one with a new screen?

You'll have to pay, first of all,( unless you have apple care) and second they'll probably ask you to back up your phone first just incase they give you a new phone