You don't need to return it, just go to at&t and say you need a new sim card. They may charge for it or give it free, it kinda depends on the shop. If they give you a normal sim, just throw it in your restored 2g (make sure it says connect to itunes) and itunes should ask you to log into your at&t account and select which number you want it on. If you got a micro sim you will need to use tape to hold it in place in the 2g sim tray. Then it should give you data plan options, select unlimited + texts for $20. After you go through all that it should be activated (you may need to wait a few hours for it to activate, i did). If they gave you a normal sim you then need to cut it down to fit in the i4's micro sim tray. If not, just put the sim in the iphone 4 and after a while you should get a text saying you were put on the correct data plan for your phone.
If any of that doesn't make any sense let me know