10-4 is a popular alternative to saying “acknowledged” or “I understood” in modern context, originating from APCO Project 14 Aural Brevity Code.
The codes are used by policemen in North America to briefly communicate common messages between each other.
“10-4” is essentially the APCO code for “OK”.
The Ten Codes or Ten Signals were developed in the United States between 1937 and 1940, by Charles Hopper.
These codes were used by policemen in the US, as well as radio enthusiasts and truckers.
“10-4” became widely popularized during the 1950’s, thanks to the television series Highway Patrol, which ran between 1955 and 1959.
The phrase was often featured in popular culture, appearing in movies, series as well as songs, such as the 1975 hit of C. W. McCall titled “Convoy” as well as the 2004 Ghostface Killa track “Beat the Clock”.