Re-Activating old iPhone 4

Hello,

I recently upgraded to iPhone 5. After about a week it had a well placed drop and the screen is now broken.

Until I can afford the $230 replacement I'd like to go back to using my old iPhone 4.

The iPhone 4 still has the old SIM card, it just needs to be activated on AT&T.

I've restored the iPhone 4 but cannot get it back on AT&T.  I assume this is because the iPhone 5 uses a different SIM card.

Is there any way to get my iPhone 4 working again?  Should I go to AT&T to have them activate it?

Thanks for the help.

--Dropsy McPhonebreak