The only 4-lug rear disc setup using Ford parts that I am aware of are the '87-88 T-bird Turbo Coupe/'93 Cobra brakes. You might try taking your rotor to your local auto parts store and asking them to pull one of those rotors and compare it to yours. Other than that, a measurement of the rotor diameter and some pics might help. The only other 4-lug rear disc setups that I am aware of are from aftermarket companies like SSBC, Wilwood, etc.