What information do I need to give to buy an iPhone 4?

I am super wary of putting my social security number online, so when AT&T asked me for the last 4 digits of my social when I signed up years ago, I gave a 4 digit number.... for the life of me, I do not recall what that number is.... Will I need it to buy an iPhone in store? I had asked a couple workers, and the answers were different each time, so Im just trying to figure it out before I waste my time hours in line, just to learn i can't get one.