Straight Talk. Uses the AT&T network, $45/month, no contract. iPhone 4 must be unlocked, which you can get done through AT&T since your iPhone 4 contract expired.
If she only wants the phone for voice the GoPhone 10¢/Minute Plan is perfect. It only charges what you use to talk. A bonus is that you can add a text messaging 'package' to co-exist with this plan if she texts more than a handful of messages on a monthly basis.