With the torque converter and pan empty, the C4 requires eight quarts of fluid. Add five quarts first, then check the dipstick reading with the car on a level surface. With fluid showing on the dipstick at or below the "ADD" mark, start the engine. With the engine at idle, check fluid level, which should be well below the ADD mark. With the engine at operating temperature, check the fluid level again. It should have expanded to the ADD mark. Add fluid one quart at a time until it reaches the FULL mark. It is suggested you not overfill, but instead add until fluid is between the ADD and FULL marks. Take a spin and get your C4 to operating temperature, then check fluid level again. Add fluid to the FULL mark. After an open road drive, examine fluid consistency and color. If it is foamy, the transmission is overfilled. It's always best to underfill, then add as necessary. If you overfill, you will have to either siphon fluid out or loosen the pan. Never check transmission fluid level cold; always check at operating temperature.
Remember, your Mustang's torque converter holds nearly four quarts, or half of the transmission's fluid capacity. If you haven't drained the torque converter, all you will need is approximately four quarts.