What's the fastest way to bring down a 4 year old child's temperature?

Depends. Fever is part of the body's defense. It makes it more difficult for the germ/virus to win the battle. Thus the fever goes when the "battle" is won. If the temp goes unusually high, e.g. 105, tepid sponging of the whole body helps without causing trouble. We now know that aspirin, given to a child sometijmes gives rise to complications that are worse than the uri/flu etc.