How To Enable iPhone 4 FaceTime Video Calls Over 3G Network Using My3G
Now that you can jailbreak iPhone 4 on iOS 4, 4.0.1 (you can unlock iPhone 4 too), you might be looking forward to install some cool jailbreak apps that make your life easy. My3G is one such app which lets you make video calls over your 3G network using iPhone 4 FaceTime application, which is otherwise not possible.
My3G works by making the FaceTime as well as other apps on your iPhone think they are on a Wifi instead of 3G. Using this app you can stuff other than FaceTime video calls, such as watch and download YouTube videos, using Skype, Fring, or other VoIP clients on your 3G network.
Here are the steps to install My3G and thus enable FaceTime video calling on your iPhone 4 over 3G:
1. First jailbreak iPhone 4 using JailbreakMe 2.0 Star (userland jailbreak) and launch Cydia on your jailbroken device.
2. In Cydia navigate to Sections –> System. Scroll down and select My3G from the list of packages.

3. Install My3G app and restart your iPhone Springboard.
You can now launch My3G app from your iPhone springboard, sign into it using a Rock ID, and start using FaceTime feature over 3G network. After an initial 9 day trial period, you will be required to pay $3.99 in order to continue using the app.
The latest release of My3G brings support for iPhone 4 along with some added features such as integration with SBSettings Toggle, ability to select 3G enabled apps on a case by case basis, and so on.

SBSettings Alternative for iPhone 4S/iPad 2, No Jailbreak Required
How to Unlock iPhone 4S on AT&T without Jailbreaking
Save SHSH Blobs of iOS 5.0.1 for iPhone 4S with TinyUmbrella
Gmail App for iPhone, iPad, iPod Touch Available for Download
Enable FaceTime over 3G in iOS 5 [Jailbreak Tweak]
iTunes 10.5.1 for Windows, Mac OS X [Direct Download Links]
Jailbreak iOS 5.0.1 with Redsn0w 0.9.9b8