I would put the convertible related sheet metal parts onto the 4 door chassis and move the VIN over. You are just repairing the car so not a problem. Start drilling some spot welds and get er done.
Moving a VIN to another vehicle is ILLEGAL and can buy jail time if proven. I know if later I bought and paid a convertible price to you and found you swapped a VIN to a 4 door I would follow it through court to prosecute.