However, it is possible to enable it on iPhone 4 by hacking one of the system files.
Here are the step by step instructions to enable the feature natively on your jailbroken iPhone 4:
Step 1: Download iFile for iPhone from Cydia or iExplorer for Mac and Windows
Step 2: Browse to /System/Library/CoreServices/SpringBoard.app/
Step 3: Open the file named N90AP.plist (please take a backup of your iPhone before making any changes so you can restore it from the backup if you hit any issue)
Step 4: Below the <dict> line, under <key>capabilities</key>, add in <key>3Gvenice</key><true/>
Step 5: Save the changes.
That’s it, you should now be able to make FaceTime calls over 3G.
As always, let us know how it goes.