I am looking to switch over to the iPhone 4 today as a new AT&T costumer. Just a few concerns about a future family plan upgrade are holding me back. When the time comes, we are looking to add 3 additional lines. Come February, when our T-Mobile contract ends, my mother would also like to purchase an iPhone 4 but my father and brother would like to use their unlocked iPhones. How would this work out? Am I able to just purchase one additional phone in February while adding on 3 extra lines? This will total 4 lines. Thank you in advance for any help!