You won't be able to get a new one, probably refurbished, but it would be an OOW replacement if something is wrong with the phone. I believe the OOW price for the iPhone 4 is $149USC. What is wrong with the phone that you want to replace it, maybe someone here can help you get it working again.
Yes. But the program is not a trade-in program. It's an exchange program for non-functioning or damaged devices. If your is working they may choose not to exchange it. You'll have to ask what they will do for you.