An iPhone will only be sold in store if it is going on an active AT&T iPhone plan. If you pay the full no contract price, the phone will still only be sold if it is sold on an active iPhone plan.
The iPhone 4 can only be activated the first time on the phone number that it was sold on. When activated, it asks for the phone number it was sold on, the last 4 of the SS# and the zip. Once it's activated, it can then be used on a prepaid sim.
You would have to buy a used iPhone that has already been activated for this to work.