you have to command your program  to do so, and if you don't want to use a config file, you might be able to facilitate an IPC mechanism, so a client or commander program command the logger to change its behaviour.

provide an interface within the main application (that uses log4j) and let an admin tool to communicate with it over that interface to modify the debugging level of the main app.

unless log4j has some built-in facility, the above could be a solution. maybe dig into apache log4j document / mailing list for more info