Im not sure if this is what you are typing about or not, but a 4 hp gain at the rear wheels absolutely does not equal 4hp used to make that. If that were the case, then cars would all be making the same bhp per litre from each manufacturer. There are other factors here as he stated in his post. Like this, if an engine makes 100 hp, you are saying that it uses 100hp to make that hp thus making 0 at the crank and unable to power its self. Energy can not be created or destroyed is ONLY HALF of that equasion. It CAN be converted and used in a different manner than which it was used. A car crash for example will be ultimately measured in miles per hour at impact. This can be calculated (for simplicity sake a simple car into a parked car) by measuring skid, crush, movement of the vehicle it hit, and post impact distance of the car that did the striking. You will NEVER account for all of the energy used. You can not account for sound and light which as miniscule as they may be, can not be caputred by the eye/ear alone. With that, you would have to use different formulas to incorporate each of the aformentioned things and then convert them to like/same values to get the speed of the striking vehicle.
Example: to measure crush, you need to find kenetic energy used to smash the vehicles. Different ways to do this I won't go into. To calculate the skid of the vehicles, you take the friction surfaces, distances, etc to get a speed. Then to essentually use the common denominator, you convert them to the like valuse and combine them to get the initial speed.
That said, you HAVE to have a gain in certain things to make power. I understand that 1 gal of petrol does not equal 1hp. But to change the chemical composition of the fuel burned and stating there is no gain is like saying nitrous does not make a vehicle faster.
SWS