AT&T calls this an Upgrade Swap. If you don't want to go to AT&T, make sure that you don't activate the Micro-SIM, as it can only be used once.
Yes but you will need to call Customer Care first as the IMEI of the iPhone 4 is parked on the line that it was ordered against. The Care rep will need to move the IMEI of the iPhone 4 to your line and may ask you for the ICCID of the Micro-SIM. Once all this is done you can use iTunes to activate the iPhone 4. If the other line's phone number still shows up in iTunes, the Care rep has not completed the move of the information.
Here is the official AT&T answer:
http://www.wireless.att.com/learn/articles-resources/iphone4upgradesupport.jsp
Dave