[Police] off topic - "10-4"

Hi it's time for another movie question (thanks to all who cared to answer my "Marco Polo"-question...). I just saw "Bringing out the dead" with Nicolas Cage. He plays a paramadic in New York. When he and his partners are called to an emergency they always say "10-4" to show that they understood what was said. Now where does that "10-4" come from? Thanks Dietmar