What does the 10 in 10-4 mean?
It doesn't really mean anything, just read this paragraph from Wikipedia. "Ten-codes were developed in the 1940s at a time when police radio channels were limited, to reduce use of speech on the radio. Credit to the originator goes to Charles "Charlie" Hopper. He was the Communications Director at the Illinois State Police, District 10, located in Pesotum, Illinois. Hopper was involved in radio for many years and saw a need to abbreviate radio transmissions on State Police bands. Experienced radio operators know that the first syllable of a transmission is frequently not going to be understood, but is a necessary part of "tuning in"; hence preceding every code with "ten" allows a better chance of understanding the critical portion. Also the radios of the day were based on vacuum tubes, with a small motor-generator (called a dynamotor) used to generate the high voltage (300-600 volts, depending on the type of radio) needed to operate the transmitter, and the dynamotor took 1/10 to 1/4 of a second to "spin up". The officers were trained to push the microphone button, and wait a moment, then talk, but sometimes they would forget and preceeding every code with "ten-" gave the radio transmitter time to come up to full power. Ten-codes were later adapted for use by CB radio enthusiasts before its pop culture explosion in the late 1970s. The tremendous popularity of the 1975 Convoy song by C.W. McCall depicting droll conversation among CB-communicating truckers put several phrases, such as 10-4 for "understood" and what's your twenty? (10-20) for "where are you?" into common and enduring use in American English. The song was followed by a 1978 movie Convoy which further entrenched the use of ten-codes in casual conversation." -Wikipedia