As of 4D 20, there is now a command that will set dufferent options to be used upon the next HTTP request triggered by HTTP Get or HTTP Request. Specifically, you can record the HTTP client request log file with the command below:

HTTP Set Option(HTTP client log; HTTP enable log with all body parts)

With this command, a file named "4DHTTPClientLog.txt" will be stored in the "Logs" folder of the application. Whole requests, including headers, are logged. The constant HTTP enable log with all body parts logs the body parts of the request and response as well.

More options and constants for this command could be found in the documentation here:
https://doc.4d.com/4Dv20/4D/20.3/HTTP-SET-OPTION.301-6867634.en.html