This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
This says that the file is in the current working directory, which, chances are, isn't your WEB-INF/classes directory.
I don't explicitly initialize log4j. I just put the config files in that directory and let it find them on it's own by traversing the classpath.
If you need to initialize it yourself, look at either getRealPath (which sometimes will return the path of a resource within your web app) or, better, look into getResourceAsStream and learn how to stream the file to log4j.