How would I make my iPhone 4 do FaceTime off of my cellular date and not wifi?

Settings > WiFi = "Off" when using FaceTime.

Go to Settings > Cellular

Scroll down and make sure that FaceTime is enabled.

Also if you own an iPhone 4 you will now be able to use it over cellular data, or if your carrier restricts it.

Reffer to:

http://support.apple.com/kb/HT4319

Hope it helps!

The iPhone 4 does not support FT over cel data. You can only do it over wifi.