For those of us old enough to remember the CB radio craze of the 1970s, 10-4 was heard on the radio, Citizen Band radio, and in the hit song, Convoy.
10-4 means ok.
From the CB Chatter that precedes the lyrics:
[CB Radio Chatter]
Yeah, breaker one-nine
This here’s the Rubber Duck
You got a copy on me Pig Pen, c’mon?
Uh, yeah, Ten-Four Pig Pen, fer sure, fer sure
By golly it’s clean clear to Flag Town, c’mon
Yeah, its a big Ten-Four there, Pig Pen
Yeah, we definitely got the front door, Good Buddy
Mercy sakes alive, looks like we’ve got us a convoy