"10-4" is one of the so-called "ten-codes" or "10-codes" that were developed in the 1930s for use in radio communication. These codes were originally designed to convey specific information with brevity and clarity, especially in situations where radio communication could be hindered by static or other interference.

The history of the ten-codes traces back to Charles "Charlie" Hopper, who was the Communications Director for the Illinois State Police. Recognizing the need for a standardized and concise way to communicate over the radio, Hopper developed the ten-codes for this purpose.

"10-4" specifically means "acknowledged" or "message received." Over time, thanks in part to their use in popular culture (such as in movies, TV shows, and songs), some of these codes, particularly "10-4," have entered into general slang and are recognized even outside of radio communication circles.

However, it's worth noting that with advancements in communication technology and the desire for clear, unambiguous communication (especially in emergency situations), many public safety agencies have moved away from using ten-codes in favor of plain language communication.