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Hi, acetone is a good cleaner for fuel tanks. If the car has been sitting for 3 - 4 years you really should remove the tank from the car in order to clean it. Your biggest concern will be rust inside the tank. You can pour in a gallon of acetone and work it around the tank and see if it comes clean. After that I would just use a laundry detergent like Tide mixed with water to clean the inside. Rinse the tank thoroughly and let it dry in the sun. If the tank is really bad I would recommend contacting some radiator shops in your area and see if any of them are experienced in the urethane coating process for the inside of the tank. After a thorough cleaning of the tank they will coat the inside of the tank with urethane to seal in any remaining rust and loose particles.
Let me know if you have any questions.