However, there is one way in which you could get FaceTime on your device, even in iOS 5.
The app is exclusively designed for mobile devices running in iOS with a forward-facing camera but by hacking one of the system files FaceTime can be installed on iPhone 4 as well.
Here are the steps to follow: first, you have to download iFile for iPhone from Cydia or iExplorer for Mac and Windows. Then you browse to /System/Library/CoreServices/SpringBoard.app/.
Open the file named N90AP.plist and add in 3Gvenice under capabilities. Next you click save and you’re done.
FaceTime works by connecting an iPhone 4 or newer iPhone, an iPod touch, an iPad 2, or a computer with Mac OS X to another similar device.