Green’s Dictionary of Slang

1. message received and understood; often ext. as that’s a big ten-four.

1962		D. Dempsey ‘Lang. of Traffic Policemen’ in AS XXXVII:4 272: ten-four, v. To understand a message. From the radio code 10-4, meaning I receive you clearly.
1969		D. Pendleton Executioner (1973) 180: ‘Wait for us there.’ ‘Ten-four.’ Lyons threw down the mike.
1976		Manchester Guardian Weekly 30 May 1: ‘Ten four old buddy, see you on the flip’ they yell as their trucks pass in a roar of spray and fumes.
1987		‘Joe Bob Briggs’ Joe Bob Goes to the Drive-In 113: A big 10-4 on Temple, pardner.
1996		P. Cornwell Cause of Death (1997) 25: Ten-four. You’re breaking up.
2004		Mad mag. Dec. 28: That’s a big ten-four on slappin’ the big greezy with a shovel.

2. in fig. use, ready, prepared.

1990		S. Morgan Homeboy 5: She was a big Ten Four for the rest of the evening.
1996		P. Cornwell Cause of Death (1997) 25: Everything ten-four?
2000		M. Collins Keepers of Truth 69: More of the cop talk we all used these days. ‘Ten-four, Pete.’.