I believe the motor is the same as a Q or at least I always thought they were. The 28 and up all appeared to have the same motor. By the way my C4 came with a brake. It doesn't coast as long as a Q but would make you noticed pulling up to a highway accident and not having time to brake it.
It's not plastic explosive. A C4B is a Federal siren that is larger than the model 28 but smaller than a Q. It is a coaster type and has a sound similar to a Super Chief charged and drifting. Of course it is a typical Federal high pitch siren that would break glass in Chinatown. It also has a brake. Not a bad siren but was hard to fit on station wagons and coaches other than Cadillacs.