The term 10-4, like many other codes, refers to a distinctive communication lingo adopted by users of Citizens’Band (CB) radio – primarily among truckers hauling freight in the United States throughout the 1970s and 80s.
The code 10-6 means I’m busy at the moment, but stand by and 10-13 is a request to get information about the weather and road conditions. When a CB radio message asks ‘what’s your 10-20?’ they are asking for your location – where are you?