If you have been having trouble using FaceTime with your iPhone 4, it could be a firewall problem.  I am using a Linksys router, and mine 'just worked'. If you are having issues, open up these ports on your router.

From Apple's FaceTime support document, they say:

If the Wi-Fi network router that you are connected to uses a firewall or security software to restrict Internet access, contact the network administrator and reference this technical article. To use FaceTime on a restricted Wi-Fi network, port forwarding must be enabled for ports 53, 80, 443, 4080, 5223, and 16393-16472 (UDP).

The Wi-Fi network administrator can refer to their router, firewall, or security software documentation for information on configuring port forwarding.

If you encounter issues using a Wi-Fi network, use standard Wi-Fi network troubleshooting to resolve interference and other issues.

To test, give Apple a call and test FaceTime with them. 1-888-FACETIME  (not a joke, they really have reps to FaceTime with you, during their normal business hours).