I currently have an iPhone 4, and I recently purchased the Lumia 900 from eBay. I have the $30 unlimited data iPhone plan. If I go into AT&T to get a 4G micro sim will I be able to use that new sim in my iPhone 4 and still get 3G data to it (If I decide to switch back and forth with the phones)?
You should not have any problems except that you need to call aTT each time you make the switch. The plan you currently have tells att how to support your iPhone on the back-end with things such as visual voice mail, etc.... Your new phone will also have certain features supported on the back-end that the phone needs to allow you to fully take advantage of its features.
For service or support questions including existing order status, call: Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/