I'll assume you're speaking of the output file in this answer.
In the log4j.properties file, you identify the location of the output as follows:
log4j.appender.fileOBTS.File=C:/Projects/obts/ServerOutput.html
In this example, I'm outputting to the html format and the 'fileOBTS' is the name of the category defined earlier on in the properties file.
Hope this is what you were looking for.