why do cops say "10-4"?

Answer 1

Because some police frequencies are (or used to be, we have non scannable digital signals) scannable (sp?) police use "ten code" to make it difficult for bad guys/media/general public to understand what is being said. It also helps in cutting down on the length of radio transmissions so that there is more air time for all officers.
Example of ten codes we use

10-1 : What? didn't get that
10-2 : That was loud and clear
10-3 : Stop transmitting on this channel, switch to another
10-4 : okay, acknowledged

Different police services use different ten codes, they are not universal (except for maybe the 10-4).
For example, the answer above mine gives a link to a site that lists ten code, in it it has
10-16 as a domestic disturbance and we use 10-32 (which is man with gun on that list)

Answer 2

It means acknowlegded or understood.
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 Hopper. He was the Communications Director at the Illinois State Police, District 10, located in Urbana, Illinois. Charles "Charlie" was involved in radio for many years and saw a need for a short cut for radio transmission on the State Police airways.

Answer 3

Different police agencies use different codes as some of your answers show. We say 10-4 instead of "I understand your transmission and I will take care of it".
By the same token, my brother who is a police officer in Germany of all places, says "richtig" at the end of a transmission, meaning "Correct".
I always thought that sounded kind of stupid, but he does his job and I did mine for 21 years before I retired.

Answer 4

Not only cops, but many CBers, and amateur radio operators use the "10" system as a way to shorten conversation on the airwaves.

Answer 5

when talking on the police radios, things can get a bit garbled.

if someone says 'ok' it can sound like a lot of different thing.

10-4 is a bit easier to hear and sounds like less.

in commercial aviation they also use more formal wording for communication as well - for the same reasons.

Answer 6

It's code for ok. They call them 10 codes and they are used to get the meaning of your conversation across in the shortest amount of time. You are suppose to keep your radio transmissions short not like a phone call. Someone else on the frequency might have an emergency and they can't get on the radio because someone is giving the Gettysburg Address.

Answer 7

10-4 is just an "acknowledgement", basically saying you understand the call and the response and instructions.

Answer 8

it's a confirmation code. it's a more professional way of saying "okay".

Answer 9

It is an acknowledgement that the radio message is received ok, and understood.

Answer 10

bcuz they can either say 10-4 or ok......its just basically sayin u get it