Your post says you have an iPhone 4. Therefore, you cannot use cellular data for FaceTime, period. This is from the iPhone User Manual:
With iPhone 4 or later, you can make a video call to someone with a Mac or other iOS device that supports FaceTime. The FaceTime camera lets you talk face-to-face; switch to the iSight camera on the back to share what you see around you.
Note: On iPhone 3GS or iPhone 4, you need a Wi-Fi connection to the Internet. On iPhone 4S or later, you can also make FaceTime calls over a cellular data connection. Cellular data charges may apply. To turn off FaceTime using cellular data, go to Settings > General > Cellular.