No, there is no cydia tweak for that because it's stock in the standard iOS - it's called AirPlay. And yes, it mirrors if you want it to.
The reason it's not on the iP4 is hardware related I reckon...
The reason it's not on the iP4 is hardware related I reckon...
Edit to add... there are directions out there that involve changing an entry in a plist file and requires use of Apple VGA Adapter or Apple Digital AV Adapter for mirroring.
Enable Video Mirroring on iPad 1 and iPhone 4 with a Simple Hack