Step 1: Jailbreak iOS 4.3 GM running on your iPhone 4 using PwnageTool 4.2.
Step 2: Once you’re jailbroken, launch Cydia and install OpenSSH.
Step 3: Now you’re going to SSH into your device (by using clients such as Cyberduck for Mac or WinSCP for Windows), and then navigate to the following directory: System>Library>CoreServices>SpringBoard.app>N90.plist
Step 4: Using a plist editor such as TextWrangler, edit the N90.plist file.
Step 5: Add a Boolean statement “multitasking-gestures” and set its value to “True”.
Step 6: Save everything you’ve edited, restart the Springboard.
Step 7: That’s it, you’re all done! You should now see the Multitouch Gestures option in the Settings.app file on your iPhone 4. Just tap to turn it on.