>>OK but the dynamo produces 100 Volts in DC and 400 Watts with 4000 Amps. How did they turn 4000A in 1A? <<
You're still confusing the terminology. Voltage only addresses electromotive force, or what is the electrical equivalent of pressure. Amperage deals with the amount of electricity (The volume) you're getting.
Just because you have....say...a 20 amp circuit doesn't mean that everything which is hooked up to is going to pull that much.
Somehow, we're not communicating here. Suppose, as you say, we have a generator rated at 100V, 400kW, 4000A. Just for grins, let's suppose that a single 100-W, 100-V lamp is hooked up to that generator (and there are no other loads). What will be the current supplied by the generator? ANSWER: 1A. A generator rated at 400kW and 4000A can produce anywhere between 0-400kW and 0-4000A, depending upon the connected load. You might try Googling "amps vs volts vs watts".
Here's another thought. Think about a car with a 300-hp engine. (Horsepower is somewhat akin to rated current (amps) and power (kW) of an electric generator.) Now, when you are stopped at stop-light, and the engine is just idling, supplying only the power needed for the car's radio and lights, how does that 300 hp get "turned into" (to use your term) a mere horsepower or two?