well a stock cb will pull 5w by itself so.....
Then it isn't stock.. or get a better wattmeter as most of the cb stuff is barely accurate by plus or minus 20%...
I can link you to all the documents on the FCC website but its a lot of reading and a lot of terms, like ERP and antenna gain figures. But the bone stock radio isn't supposed to be more than 4w of output power if checked with a proper 50 ohm dummy load and an accurate wattmeter. I prefer a Bird 43 myself as its the industry standard.
Not to mention the fact that someone could have given it a peak and tune to put it over 4w. The rf transistors used in most CB's are capable of more, but they turn down the output to stay within compliance, all a peak and tune is adjusting the radio to get the max out with the transistor in use, or even replacing it. Most commonly tuners will pull the stock transistor and toss in a Toshiba 12w transistor.
Trust me i know a lot about it, cause i have converted a few CB's to legal 10m ham band use. Generally going to about 25w by rebuilding the PA section.