FYI, it is not an iPhone 4G. It is an iPhone 4.
All iPhone models prior to the iPhone 4 use a regular size SIM card. The iPhone 4 uses a micro-SIM card.
Get a new SIM card at an AT&T corporate store for the iPhone 4. There will be no charge for this and the store can activate the iPhone 4 with your wife's line/number as well.