A code used by CB owners possibly since the dawn of amateur radio, the code 10-4 simply means "Message received". Since the 70s the code system have been popular in law-enforcement circles then evolved to what it is today. Nowadays when people use these codes, they are mostly refering to the police radio code system, which is more detailed then its original version, so I'am only listing the police code system here rather than the out dated CB code version.

Please note that these code vary from one country to another or even used differntly in differnt state/province in the same country

10-0 Use Caution
10-1 You are being received Poorly
10-2 You are being received Clearly
10-3 Stop Transmitting
10-4 O.K.
10-5 Relay Message
10-6 Busy with Call
10-7 Out of service, (completely)
10-7b Out of service (personal)
10-7c Out of service (court)
10-7od Out of service (off duty)
10-8 In service
10-8ot In service (over time)
10-9 Repeat last message
10-10 Out of service, Radio on
10-11 Give F.C.C. Call Sign/Dispatching too fast
10-12 Visitors or officials present
10-13 Advise weather and road conditions
10-14 Convoy or escort detail
10-15 Prisoner in custody
10-15m Prisoner in custody (mental case)
10-16 Pick up prisoner
10-17 Pick up papers
10-18 Complete present assignment ASAP/Get there ASAP
10-19 Return to office
10-20 Your location
10-21 Call by telephone
10-21A Phone home, my ETA is _____
10-21B Call your home
10-22 Cancel last message/Take no...

in trey songz song 10-4 off of his latest mixtape lemmeholdatbeat he talks about being in the club and having girls looking for him and tryin to approach him. he says...

A girl being a rating of 10 from afar and a 4 from up close.. Basically an ugly girl up close and hot from afar